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The Internet Protocol (IP) is a packet-based protocol used to exchange data over computer networks. IP handles addressing, fragmentation, reassembly, and protocol demultiplexing. It is the foundation on which all other Internet protocols, collectively referred to as the Internet Protocol suite, are built. IP is a network-layer protocol that contains addressing information and some control information that allows data packets to be routed.
The Transmission Control Protocol (TCP) is built upon the IP layer. TCP is a connection-oriented protocol that specifies the format of data and acknowledgments used in the transfer of data. TCP also specifies the procedures that the computers use to ensure that the data arrives correctly. TCP allows multiple applications on a system to communicate concurrently because it handles all demultiplexing of the incoming traffic among the application programs.
Use the commands in this chapter to configure and monitor IP networks. For IP protocol configuration information and examples, refer to the "Configuring IP" chapter of the Router Products Configuration Guide.
Use the access-class line configuration command to restrict incoming and outgoing connections between a particular virtual terminal line (into a Cisco device) and the addresses in an access list. The no access-class command removes access restrictions on the line for the specified connections.
access-class access-list-number {in | out}| access-list-number | Integer from 1 through 99 that identifies a specific access list of Internet addresses. |
| in | Restricts incoming connections between a particular Cisco device and the addresses in the access list. |
| out | Restricts outgoing connections between a particular Cisco device and the addresses in the access list. |
No access lists are defined.
Line configuration
Remember to set identical restrictions on all the virtual terminal lines because a user can connect to any of them.
To display the access lists for a particular terminal line, use the show line EXEC command and specify the line number.
The following example defines an access list that permits only hosts on network 192.89.55.0 to connect to the virtual terminal ports on the router:
access-list 12 permit 192.89.55.0 0.0.0.255
line 1 5
access-class 12 in
The following example defines an access list that denies connections to networks other than network 36.0.0.0 on terminal lines 1 through 5:
access-list 10 permit 36.0.0.0 0.255.255.255
line 1 5
access-class 10 out
A dagger (+) indicates that the command is documented in another chapter.
show line +
Use the access-list global configuration command to create or remove an access list and control access to it. Use the no access-list command to delete the entire access list.
access-list access-list-number {permit | deny} source [source-mask]| access-list-number | Integer from 1 through 99 that you assign to identify one or more permit/deny conditions as an access list. Access list 0 (zero) is predefined; it permits any address and is the default access list for all interfaces. |
| permit | Permits access for matching conditions. |
| deny | Denies access to matching conditions. |
| source | Compares the source address being tested to this value. It is a 32-bit quantity written in dotted-decimal format. See the examples later in this section. |
| source-mask | (Optional) 32-bit quantity written in dotted-decimal format. Address bits corresponding to wildcard mask bits set to 1 are ignored in comparisons; address bits corresponding to wildcard mask bits set to zero are used in comparisons. See the examples later in this section. |
The access list defaults to an implicit deny statement for everything that has not been permitted.
Global configuration
Plan your access conditions carefully and be aware of the implicit deny.
You can use access lists to control the transmission of packets on an interface, to control virtual terminal line access, and to restrict contents of routing updates.
Use the show access-lists EXEC command to display the contents of all access lists.
The following example of a standard access list allows access for only those hosts on the three specified networks. It assumes that subnetting is not used; the masks apply to the host portions of the network addresses. Any hosts with a source address that does not match the access list statements will be rejected.
access-list 1 permit 192.5.34.0 0.0.0.255
access-list 1 permit 128.88.1.0 0.0.255.255
access-list 1 permit 36.0.0.0 0.255.255.255
! (Note: all other access implicitly denied)
To specify a large number of individual addresses more easily, you can omit the address mask; that is, all zeros from the access-list command. Thus, the following two configuration commands are identical in effect:
access-list 2 permit 36.48.0.3
access-list 2 permit 36.48.0.3 0.0.0.0
show access-lists
Use the extended access-list global configuration command to create or remove an extended access list. Use the no access-list command to delete the entire extended access list.
access-list access-list-number {permit | deny} protocol source source-mask destination destination-mask [operator operand] [established] no access-list access-list-number| access-list-number | Integer from 100 through 199 that you assign to identify one or more extended permit/deny conditions as an extended access list. Note that a list number in the range 100 through 199 distinguishes an extended access list from a standard access list. |
| permit | Permits access to matching conditions. |
| deny | Denies access to matching conditions. |
| protocol | One of the following protocols: ip, tcp, udp, icmp, igmp, gre, or igrp or an integer in the range of 0 through 255 representing an IP protocol number. Use the keyword ip to match any Internet protocol, including TCP, UDP, and ICMP. |
| source | Internet source address in dotted-decimal format. Used in conjunction with source masks. |
| source-mask | Mask of source address bits in dotted-decimal format. The source and source-mask arguments are used to match the source address of a packet. |
| destination | Internet destination address in dotted-decimal format. Used in conjunction with destination masks. |
| destination-mask | Mask of destination address bits in dotted-decimal format. The destination and destination mask arguments are used to match the destination address of a packet. |
| operator | (Optional) Compares destination ports. Note that the ip and icmp protocol keywords do not allow port distinctions. Possible operands include lt (less than), gt (greater than), eq (equal), and neq (not equal). |
| operand | (Optional) Decimal destination port to compare. Note that the ip and icmp protocol keywords do not allow port distinctions. |
| established | (Optional) For the TCP protocol only: to indicate an established connection. A match occurs if the TCP datagram has the ACK or RST bits set. The nonmatching case is that of the initial TCP datagram to form a connection. |
An extended access list defaults to an implicit deny statement for everything that has not been permitted.
Global configuration
You can use access lists to control the transmission of packets on an interface, to control virtual terminal line access, and to restrict contents of routing updates. The router stops checking the extended access list after a match occurs.
Fragmented IP packets, other than the initial fragment, are immediately accepted by any extended IP access list.
In the following example, the Ethernet network is a Class B network with the address 128.88.0.0, and the mail host's address is 128.88.1.2. The keyword established is used only for the TCP protocol to indicate an established connection. A match occurs if the TCP datagram has the ACK or RST bits set, which indicate that the packet belongs to an existing connection.
access-list 102 permit tcp 0.0.0.0 255.255.255.255 128.88.0.0 0.0.255.255 established
access-list 102 permit tcp 0.0.0.0 255.255.255.255 128.88.1.2 0.0.0.0 eq 25
interface ethernet 0
ip access-group 102 in
ip access-group
show access-lists
Use the arp global configuration command to install a permanent entry in the ARP cache. The router uses this entry to translate 32-bit Internet Protocol addresses into 48-bit hardware addresses. Use the no arp command to remove the specified entry from the ARP cache.
arp ip-address hardware-address type [alias]| ip-address | IP address in dotted-decimal format corresponding to the local data link address. |
| hardware-address | Local data link address (a 48-bit address). |
| type | Encapsulation description. This is typically the arpa keyword for Ethernet and is always snap for FDDI and Token Ring interfaces. |
| alias | (Optional) Indicates that the router should respond to ARP requests as if it were the owner of the specified address. |
No entries are permanently installed in the ARP cache.
Global configuration
Because most hosts support dynamic resolution, you generally do not need to specify static ARP cache entries.
To remove all nonstatic entries from the ARP cache, use the clear arp-cache privileged EXEC command.
The following is an example of a static ARP entry for a typical Ethernet host:
arp 192.31.7.19 0800.0900.1834 arpa
clear arp-cache
Use the arp interface configuration command to control the interface-specific handling of IP address resolution into 48-bit Ethernet, FDDI, and Token Ring hardware addresses. Use the no arp command to selectively disable the specified interface encapsulation type.
arp {arpa | probe | snap}| arpa | Standard Ethernet-style ARP (RFC 826) |
| probe | HP Probe protocol for IEEE-802.3 networks |
| snap | ARP packets conforming to RFC 1042 |
Standard Ethernet-style ARP
Interface configuration
Unlike most commands that take multiple arguments, arguments to the arp command are not mutually exclusive. Each command enables or disables a specific type of ARP. For example, if you enter the arp arpa command followed by the arp probe command, the router would send three (two for probe and one for arpa) packets each time it needed to discover a MAC address.
The arp probe command allows the router to use the Probe protocol (in addition to ARP) whenever it attempts to resolve an IEEE-802.3 or Ethernet local data link address. The subset of Probe that performs address resolution is called Virtual Address Request and Reply. Using Probe, the router can communicate transparently with Hewlett-Packard IEEE-802.3 hosts that use this type of data encapsulation.
The show interfaces EXEC command displays the type of ARP being used on a particular interface. To remove all nonstatic entries from the ARP cache, use the clear arp-cache privileged EXEC command.
The following example enables probe services:
interface ethernet 0
arp probe
clear arp-cache
show interfaces
Use the arp timeout interface configuration command to control the number of seconds an ARP cache entry will stay in the cache. Use the no arp timeout command to restore the default value.
arp timeout seconds| seconds | Value used to age an ARP cache entry related to that interface. A value of 0 (zero) seconds sets no timeout; then the cache entries are never cleared. |
14400 seconds (4 hours)
Interface configuration
This command is ignored when issued on interfaces that do not use ARP. The show interfaces EXEC command displays the ARP timeout value. The value follows the "Entry Timeout:" heading, as seen in this sample show interfaces display:
ARP type: ARPA, PROBE, Entry Timeout: 14400 sec
The following example illustrates how to set the ARP timeout to 12000 seconds to allow entries to time out more quickly than the default:
interface ethernet 0
arp timeout 12000
show interfaces
Use the clear arp-cache privileged EXEC command to remove all dynamic entries from the ARP cache, to clear the fast-switching cache, and to clear the IP route cache.
clear arp-cacheThis command has no arguments or keywords.
Privileged EXEC
The following example removes all dynamic entries from the ARP cache and clears the fast-switching cache:
clear arp-cache
arp (global)
arp (interface)
Use the clear host privileged EXEC command to remove one or all entries from the host name-and-address cache.
clear host {name | *}| name | Particular host entry to remove. |
| * | Removes all entries. |
Privileged EXEC
The host name entries will not be removed from NVRAM, but will be cleared in running memory.
The following example clears all entries from the host name-and-address cache:
clear host *
show hosts
ip host
Use the clear ip accounting privileged EXEC command to clear the active database when IP accounting is enabled. Use the clear ip accounting checkpoint command to clear the checkpointed database when IP accounting is enabled.
clear ip accounting [checkpoint]| checkpoint | (Optional) Clears the checkpointed database. |
Privileged EXEC
You can also clear the checkpointed database by issuing the clear ip accounting command twice in succession.
The following example clears the active database when IP accounting is enabled:
clear ip accounting
ip accounting
ip accounting-list
ip accounting-threshold
ip accounting-transits
show ip accounting
Use the clear ip route privileged EXEC command to remove one or more routes from the IP routing table.
clear ip route {network [mask] | *}| network | Network or subnet address to remove. |
| mask | (Optional) Subnet address to remove. |
| * | Removes all routing table entries. |
All entries are removed.
Privileged EXEC
The following example removes a route to network 132.5.0.0 from the IP routing table:
clear ip route 132.5.0.0
Use the clear ip sse privileged EXEC command to cause the route processor to recompute the SSE program for IP on the Cisco 7000 series.
clear ip sseThis command has no arguments or keywords.
Disabled
Privileged EXEC
The silicon switching engine (SSE) is on the Silicon Switch Processor (SSP) board in the Cisco 7000.
This command also updates the SSE cache for IP.
The following example causes the route processor to recompute the program for IP:
clear ip sse
Use the clear sse privileged EXEC command to reinitialize the route processor on the Cisco 7000 series.
clear sseThis command has no arguments or keywords.
Disabled
Privileged EXEC
The silicon switching engine (SSE) is on the Silicon Switch Processor (SSP) board in the
Cisco 7000.
The following example causes the route processor to be reinitialized:
clear sse
Use the dnsix-dmdp retries global configuration command to set the retransmit count used by the DNSIX Message Delivery Protocol (DMDP). Use the no dnsix-dmdp retries command to revert to the default number of retries.
dnsix-dmdp retries count| count | Number of times DMDP will retransmit a message. An integer from 0 through 200. |
Retransmits messages up to 4 times, or until acknowledged
Global configuration
The following example sets the number of times DMDP will attempt to retransmit a message to 150:
dnsix-dmdp retries 150
dnsix-nat authorized-redirection
dnsix-nat primary
dnsix-nat secondary
dnsix-nat source
dnsix-nat transmit-count
Use the dnsix-nat authorized-redirection global configuration command to specify the address of a collection center that is authorized to change the primary and secondary addresses of the host to receive audit messages. Use the no dnsix-nat authorized-redirection command to delete the entry.
dnsix-nat authorized-redirection ip-address| ip-address | IP address of the host from which redirection requests are permitted |
An empty list of addresses
Global configuration
Use multiple dnsix-nat authorized-redirection commands to specify a set of hosts that are authorized to change the destination for audit messages. Redirection requests are checked against the configured list, and if the address is not authorized the request is rejected and an audit message is generated. If no address is specified, no redirection messages are accepted.
The following example specifies that the address of the collection center that is authorized to change the primary and secondary addresses is 193.1.1.1.
dnsix-nat authorization-redirection 193.1.1.1.
Use the dnsix-nat primary global configuration command to specify the IP address of the host to which DNSIX audit messages are sent. Use the no dnsix-nat primary command to delete the entry.
dnsix-nat primary ip-address| ip-address | IP address for the primary collection center |
Messages are not sent.
Global configuration
An IP address must be configured before audit messages can be sent.
The following example configures an IP address as the address of the host to which DNSIX audit messages are sent:
dnsix-nat primary 194.1.1.1
Use the dnsix-nat secondary global configuration command to specify an alternate IP address for the host to which DNSIX audit messages are sent. Use the no dnsix-nat secondary command to delete the entry.
dnsix-nat secondary ip-address| ip-address | IP address for the secondary collection center |
No alternate IP address is known.
Global configuration
When the primary collection center is unreachable, audit messages are sent to the secondary collection center instead.
The following example configures an IP address as the address of an alternate host to which DNSIX audit messages are sent:
dnsix-nat secondary 193.1.1.1
Use the dnsix-nat source global configuration command to start the audit-writing module and to define audit trail source address. Use the no dnsix-nat source command to disable the DNSIX audit trail writing module.
dnsix-nat source ip-address| ip-address | Source IP address for DNSIX audit messages |
Disabled
Global configuration
This command must be issued before any of the other dnsix-nat commands. The configured IP address is used as the source IP address for DMDP protocol packets sent to any of the collection centers.
The following example enables the audit trail writing module, and specifies that the source IP address for any generated audit messages should be the same as the primary IP address of interface Ethernet 0.
dnsix-nat source 128.105.2.5
interface ethernet 0
ip address 128.105.2.5 255.255.255.0
Use the dnsix-nat transmit-count global configuration command to cause the audit writing module to collect multiple audit messages in the buffer before sending the messages to a collection center. Use the no dnsix-nat transmit-count command to revert to the default audit message count.
dnsix-nat transmit-count count| count | Number of audit messages to buffer before transmitting to the server. Integer from 1 through 200. |
One message is sent at a time.
Global configuration
An audit message is sent as soon as the message is generated by the IP packet-processing code. The audit writing module can, instead, buffer up to several audit messages before transmitting to a collection center.
The following example configures the system to buffer five audit messages before transmitting them to a collection center:
dnsix-nat transmit-count 5
Use the ip access-group interface configuration command to control access to an interface. Use the no ip access-group command to remove the specified access group.
ip access-group access-list-number {in | out}| access-list-number | Access list number from 1 through 199 |
| in | Filters on inbound packets |
| out | Filters on outbound packets |
Entering a keyword is strongly recommended, but if a keyword is not specified, out is the default.
Interface configuration
For inbound access lists, after receiving a packet, the router checks the source address of the packet against the access list. If the access list permits the address, the router continues to process the packet. If the access list rejects the address, the router discards the packet and returns an ICMP Host Unreachable message.
For outbound access lists, after receiving and routing a packet to a controlled interface, the router checks the source address of the packet against the access list. If the access list permits the address, the router transmits the packet. If the access list rejects the address, the router discards the packet and returns an ICMP Host Unreachable message.
Access lists are applied on either outbound or inbound interfaces.
If the specified access list does not exist, all packets are passed.
When you enable outbound access lists, you automatically disable autonomous switching for that interface.When you enable input access lists on any cBus or CxBus interface, you automatically disable autonomous switching for all interfaces (with one exception; an SSE configured with simple access lists can still switch packets, on output only).
The following example applies list 101 on packets outbound from Ethernet 0:
interface ethernet 0
ip access-group 101 out
access-list (extended)
show access-lists
Use the ip accounting interface configuration command to enable IP accounting on an interface. Use the no ip accounting command to disable IP accounting.
ip accountingThis command has no arguments or keywords.
Disabled
Interface configuration
IP accounting is enabled on a per-interface basis. The IP accounting support records the number of bytes and packets switched through the system on a source and destination IP address basis. Only transit IP traffic is measured and only on an outbound basis; traffic generated by the router or terminating in the router is not included in the accounting statistics.
It does not matter whether or not IP fast switching or IP access lists are being used on that interface. The numbers will be accurate; however, IP accounting does not keep statistics if autonomous switching is enabled.
The following example enables IP accounting on interface Ethernet 0:
interface ethernet 0
ip accounting
clear ip accounting
ip accounting-list
ip accounting-threshold
ip accounting-transits
show ip accounting
Use the ip accounting-list global configuration command to specify a set of filters to control the hosts for which IP accounting information is kept. Use the no ip accounting-list command with the appropriate argument to remove this function.
ip accounting-list ip-address mask| ip-address | IP address in dotted-decimal format |
| mask | IP mask |
No filters are defined.
Global configuration
The source and destination address of each IP datagram is logically ANDed with the mask and compared with the ip-address. If there is a match, the information about the IP datagram will be entered into the accounting database. If there is no match, the IP datagram is considered a transit datagram and will be counted according to the setting of the ip accounting-transits global configuration command.
The following example adds all hosts with IP addresses beginning with 192.31 to the list of hosts for which accounting information will be kept:
ip accounting-list 192.31.0.0 255.255.0.0
clear ip accounting
ip accounting
ip accounting-threshold
ip accounting-transits
show ip accounting
Use the ip accounting-threshold global configuration command to set the maximum number of accounting entries to be created. Use the no ip accounting-threshold command to restore the default.
ip accounting-threshold threshold| threshold | Maximum number of entries (source and destination address pairs) that the router accumulates, preventing IP accounting from possibly consuming all available free memory. |
512 entries
Global configuration
The accounting threshold defines the maximum number of entries (source and destination address pairs) that the router accumulates, preventing IP accounting from possibly consuming all available free memory. This level of memory consumption could occur in a router that is switching traffic for many hosts. Overflows will be recorded; see the monitoring commands for display formats.
The default accounting threshold of 512 entries results in a maximumn table size of 12928 bytes. Active and checkpointed tables can reach this size independently.
The following example sets the IP accounting threshold to only 500 entries:
ip accounting-threshold 500
clear ip accounting
ip accounting
ip accounting-list
ip accounting-transits
show ip accounting
Use the ip accounting-transits global configuration command to control the number of transit records that will be stored in the IP accounting database. Use the no ip accounting-transits command to remove this function, resetting the value to the default.
ip accounting-transits count| count | Number of transit records that will be stored in the IP accounting database |
0
Global configuration
Transit entries are those that do not match any of the filters specified by ip accounting-list global configuration commands. If no filters are defined, no transit entries are possible.
To maintain accurate accounting totals, the router software maintains two accounting databases: an active and a checkpointed database.
The following example specifies that no more than 100 transit records are stored:
ip accounting-transit 100
clear ip accounting
ip accounting
ip accounting-list
ip accounting-threshold
show ip accounting
Use the ip address interface configuration command to set an IP address for an interface. Use the no ip address command to remove the specified address.
ip address ip-address mask| ip-address | IP address |
| mask | Mask for the associated IP subnet |
No IP address is defined for an interface.
Interface configuration
Hosts can determine subnet masks using the Internet Control Message Protocol (ICMP) Mask Request message. Routers respond to this request with an ICMP Mask Reply message.
You can disable IP processing on a particular interface by removing its IP address with the no ip address command. If the router detects another host using one of its IP addresses, it will print an error message on the console.
In the following example, 131.108.1.27 is the primary address for Ethernet 0:
interface ethernet 0
ip address 131.108.1.27 255.255.255.0
Use the ip address secondary interface configuration command to set multiple IP addresses for an interface. Use the no ip address secondary command to remove the specified addresses.
ip address ip-address mask secondary| ip-address | IP address |
| mask | Mask for the associated IP subnet |
No secondary IP addresses are defined.
Interface configuration
Hosts can determine subnet masks using the Internet Control Message Protocol (ICMP) Mask Request message. Routers respond to this request with an ICMP Mask Reply message.
Packets generated by the router always use the primary interface IP address. Therefore, all routers on a segment should share the same primary network number.
In the following example, 131.108.1.27 is the primary address and 192.31.7.17 and 192.31.8.17 are secondary addresses for Ethernet 0:
interface ethernet 0
ip address 131.108.1.27 255.255.255.0
ip address 192.31.7.17 255.255.255.0 secondary
ip address 192.31.8.17 255.255.255.0 secondary
Use the ip broadcast-address interface configuration command to define a broadcast address for an interface. Use the no ip broadcast-address command to restore the IP broadcast address to the default.
ip broadcast-address [ip-address]| ip-address | (Optional) IP broadcast address for a network |
Default address: 255.255.255.255 (all ones)
Interface configuration
The following example specifies an IP broadcast address of 0.0.0.0:
ip broadcast-address 0.0.0.0
Use the ip cache-invalidate-delay global configuration command to control the invalidation rate of the IP route cache. Use the no ip cache-invalidate-delay command to allow the IP route cache to be immediately invalidated.
ip cache-invalidate-delay [minimum maximum quiet threshold]| minimum | (Optional) Minimum time, in seconds, between invalidation request and actual invalidation. The default is 2 seconds. |
| maximum | (Optional) Maximum time, in seconds, between invalidation request and actual invalidation. The default is 5 seconds. |
| quiet | (Optional) Length of quiet period, in seconds, before invalidation. |
| threshold | (Optional) Maximum number of invalidation requests considered to be quiet. |
minimum = 2 seconds
maximum = 5 seconds, and 3 seconds with no more than zero invalidation requests
Global configuration
All cache invalidation requests are honored immediately.
This command should typically not be used except under the guidance of technical support personnel. Incorrect settings can seriously degrade network performance.
The IP fast switching and autonomous switching features maintain a cache of IP routes for rapid access. When a packet is to be forwarded and the corresponding route is not present in the cache, the packet is process-switched and a new cache entry is built. However, when routing table changes occur (such as when a link or an interface goes down), the route cache must be flushed so that it can be rebuilt with up-to-date routing information.
This command controls how the route cache is flushed. The intent is to delay invalidation of the cache until after routing has settled down, since there tend to be many route table changes clustered in a short period of time, and the cache may be flushed repeatedly, which may put a high CPU load on the router.
When this feature is enabled, and the system requests that the route cache be flushed, the request is held for at least minimum seconds. Then the system determines whether the cache has been "quiet," that is, less than threshold invalidation requests in the last quiet seconds. If the cache has been quiet, the cache is then flushed. If the cache does not become quiet within maximum seconds after the first request, it is flushed unconditionally.
Manipulation of these parameters trades off CPU utilization versus route convergence time. Note that this does not affect the timing of the routing protocols, but only of the removal of stale cache entries.
The following example sets a minimum delay of 5 seconds, a maximum delay of 30 seconds, and a quiet threshold of no more than 5 invalidation requests in the previous 10 seconds:
ip cache-invalidate-delay 5 30 10 5
ip route-cache
show ip cache
Use the ip default-gateway global configuration command to define a default gateway (router) when IP routing is disabled. Use the no ip default-gateway command to disable this function.
ip default-gateway ip-address| ip-address | IP address of the router |
Disabled
Global configuration
The router sends any packets that need the assistance of a gateway to the address you specify. If another gateway has a better route to the requested host, the default gateway sends an ICMP redirect message to the router. The ICMP redirect message indicates which local router the router should use.
The following example defines the router on IP address 192.31.7.18 as the default router:
ip default-gateway 192.31.7.18
show ip redirects
Use the ip directed-broadcast interface configuration command to enable directed broadcast-to-physical broadcast translation on an interface. Use the no ip directed-broadcast command to disable directed broadcast-to-physical broadcast translation on an interface.
ip directed-broadcast [access-list-number]| access-list-number | (Optional) Number of the access list. If specified, a broadcast must pass the access list to be forwarded. If not specified, all broadcasts will be forwarded. |
Enabled, with no list specified
Interface configuration
This feature is enabled only for those protocols configured using the ip forward-protocol global configuration command. An access list may be specified to control which broadcasts are forwarded. When an access list is specified, only those IP packets permitted by the access list are eligible to be translated from directed broadcasts to physical broadcasts.
The following example enables forwarding of IP directed broadcasts on interface Ethernet 0:
interface ethernet 0
ip directed-broadcast
ip forward-protocol
Use the ip domain-list global configuration command to define a list of default domain names to complete unqualified host names. Use the no ip domain-list command with the appropriate argument to delete a name from the list.
ip domain-list name| name | Domain name; do not include the initial period that separates an unqualified name from the domain name. |
No domain names are defined.
Global configuration
If there is no domain list, the domain name that you specified with the ip domain-name global configuration command is used. If there is a domain list, the default domain name is not used. The ip domain-list command is similar to the ip domain-name command, except that with ip domain-list you can define a list of domains, each to be tried in turn.
The following example adds several domain names to a list:
ip domain-list martinez.com
ip domain-list stanford.edu
The following example adds a name to and then deletes a name from the list:
ip domain-list sunya.edu
no ip domain-list stanford.edu
ip domain-name
Use the ip domain-lookup global configuration command to enable the IP Domain Name System-based host name-to-address translation. Use the no ip domain-lookup command to disable the Domain Name System.
ip domain-lookupThis command has no arguments or keywords.
Enabled
Global configuration
The following example enables the IP Domain Name System-based host name-to-address translation:
ip domain-lookup
ip domain-lookup nsap
ip domain-name
ip name-server
Use the ip domain-lookup nsap global configuration command to allow Domain Name System (DNS) queries for CLNS addresses. Use the no ip domain-lookup nsap command to disable this feature.
ip domain-lookup nsapThis command has no arguments or keywords.
Enabled
Global configuration
With both IP and ISO CLNS enabled on a router, this feature allows the router to dynamically determine a CLNS address given a host name. This feature is useful for the ISO CLNS ping EXEC command and when making CLNS Telnet connections.
The following example disables DNS queries of CLNS addresses:
no ip domain-lookup nsap
A dagger (+) indicates that the command is documented in another chapter.
ip domain-lookup
ping (for ISO CLNS) +
Use the ip domain-name global configuration command to define a default domain name that the router uses to complete unqualified host names (names without a dotted-decimal domain name). Use the no ip domain-name command to disable the use of the Domain Name System.
ip domain-name name| name | Default domain name used to complete unqualified host names; do not include the initial period that separates an unqualified name from the domain name. |
Enabled
Global configuration
Any IP host name that does not contain a domain name (that is, any name without a dot), will have the dot and cisco.com appended to it before being added to the host table.
The following example defines cisco.com as the default domain name:
ip domain-name cisco.com
ip domain-list
ip domain-lookup
ip name-server
Use the ip forward-protocol global configuration command to specify which protocols and ports the router will forward. Use the no ip forward-protocol command (with the appropriate keyword and argument) to remove the protocol/port. Specifying just the protocol, without the port, disables all flooding for that protocol.
ip forward-protocol {udp [port] | nd | sdns}| udp | Forward User Datagram Protocol (UDP) datagrams. See below for a list of datagrams forwarded by default. |
| port | (Optional) Destination port that controls which UDP services are forwarded. |
| nd | Forward Network Disk (ND) datagrams. This protocol is used by older diskless SUN workstations. See below for a list of datagrams forwarded by default. |
| sdns | Network Security Protocol. |
If an IP helper address is defined, UDP forwarding is enabled on default ports.
If a helper address is specified and UDP forwarding is enabled, the following datagrams are forwarded by default:
Global configuration
The following example uses the ip forward-protocol command to specify forwarding of UDP only, then defines a helper address:
ip forward-protocol udp
!
interface ethernet 1
ip helper-address 131.120.1.0
ip directed-broadcast
ip forward-protocol spanning-tree
ip forward-protocol turbo-flood
ip helper-address
To forward any broadcasts including local subnet broadcasts, use the ip forward-protocol any-local-broadcast global configuration command. To disable this type of forwarding, use the no form of this command.
ip forward-protocol any-local-broadcastThis command has no arguments or keywords.
Disabled
Global configuration
The ip forward-protocol any-local-broadcast command forwards packets similarly to how the ip forward-protocol spanning-tree command does. That is, it fowards packets whose contents are all ones (255.255.255.255), all zeros (0.0.0.0), and, if subnetting is enabled, all networks (131.108.255.255) as an example in the network number 131.108.0.0. This mechanism also forward packets whose contents are the zeros version of the all-networks braodcast when subnetting is enabled (for example, 131.108.0.0). In addition, it forwards any local subnet broadcast packets.
Use the ip forward-protocol any-local-broadcast command in conjunction with the ip forward-protocol spanning-tree command, not as a replacement for it.
Assume a router is directly connected to subnet 1 of network 131.108.0.0 and that the netmask is 255.255.255.0. The following command enables the forwarding of IP broadcasts destined to 131.108.1.255 and 131.108.1.0 in addition to the broadcast addresses mentioned in the Usage Guidelines section:
ip forward-protocol any-local-broadcast
ip forward-protocol spanning-tree
Use the ip forward-protocol spanning-tree global configuration command to permit IP broadcasts to be flooded throughout the internetwork in a controlled fashion. Use the no ip forward-protocol spanning-tree command to disable flooding of IP broadcasts.
ip forward-protocol spanning-treeThis command has no arguments or keywords.
Disabled
Global configuration
Packets must meet the following criteria to be considered for flooding:
A flooded UDP datagram is given the destination address specified by the ip broadcast-address interface configuration command on the output interface. The destination address can be set to any desired address. Thus, the destination address may change as the datagram propagates through the network. The source address is never changed. The TTL value is decremented.
After a decision has been made to send the datagram out on an interface (and the destination address possibly changed), the datagram is handed to the normal IP output routines and is therefore subject to access lists, if they are present on the output interface.
The ip forward-protocol spanning-tree command uses the database created by the bridging spanning-tree protocol. Therefore, the transparent bridging option must be in the routing software, and bridging must be configured on each interface that is to participate in the flooding in order to support this capability.
The spanning-tree-based flooding mechanism fowards packets whose contents are all ones (255.255.255.255), all zeros (0.0.0.0), and, if subnetting is enabled, all networks (131.108.255.255) as an example in the network number 131.108.0.0. This mechanism also forward packets whose contents are the zeros version of the all-networks braodcast when subnetting is enabled (for example, 131.108.0.0).
If an interface does not have bridging configured, it still will be able to receive broadcasts, but it will never forward broadcasts received on that interface, and it will never use that interface to send broadcasts received on a different interface.
If no actual bridging is desired, you can configure a type-code bridging filter that will deny all packet types from being bridged. Refer to the Transparent Bridging chapter in the Router Products Configuration Guide for more information about using access lists to filter bridged traffic. The spanning-tree database is still available to the IP forwarding code to use for the flooding.
This command is an extension of the ip helper-address interface configuration command, in that the same packets that may be subject to the helper address and forwarded to a single network can now be flooded. Only one copy of the packet will be put on each network segment.
The following example permits IP broadcasts to be flooded through the internetwork in a controlled fashion:
ip forward-protocol spanning-tree
ip broadcast-address
ip helper-address
ip forward-protocol
ip forward-protocol turbo-flood
Use the ip forward-protocol turbo-flood global configuration command to speed up flooding of User Datagram Protocol (UDP) datagrams using the spanning-tree algorithm. Use the no ip forward-protocol turbo-flood command to disable this feature.
ip forward-protocol turbo-floodThis command has no arguments or keywords.
Disabled
Global configuration
Used in conjunction with the ip forward-protocol spanning-tree global configuration command, this feature is supported over ARPA-encapsulated Ethernets, FDDI, and HDLC-encapsulated serials, but is not supported on Token Rings. As long as the Token Rings and the non-HDLC serials are not part of the bridge group being used for UDP flooding, turbo flooding will behave normally.
The following is an example of a two-port router (2E) using this feature:
ip forward-protocol turbo-flood
ip forward-protocol spanning-tree
!
!
interface ethernet 0
ip address 128.9.1.1
bridge-group 1
!
interface ethernet 1
ip address 128.9.1.2
bridge-group 1
!
!
bridge 1 protocol dec
ip forward-protocol
ip forward-protocol spanning-tree
Use the ip gdp gdp interface configuration command to configure the router discovery feature using the Cisco Gateway Discovery Protocol (GDP) routing protocol. Use the no ip gdp gdp command to disable this feature.
ip gdp gdpThis command has no arguments or keywords.
Disabled
Interface configuration
IP routing must be disabled before you can configure this feature.
The following example configures router discovery using GDP on the Ethernet 0 interface:
interface ethernet 0
ip gdp gdp
Use the ip gdp igrp interface configuration command to configure the router discovery feature using the Cisco Interior Gateway Routing Protocol (IGRP) routing protocol. Use the no ip gdp igrp command to disable this feature.
ip gdp igrpThis command has no arguments or keywords.
Disabled
Interface configuration
IP routing must be disabled before you can configure this feature.
The following example configures router discovery using IGRP on the Ethernet 1 interface:
interface ethernet 1
ip gdp igrp
Use the ip gdp irdp interface configuration command to configure the router discovery feature using the ICMP Router Discovery Protocol (IRDP). Use the no ip gdp irdp command to disable this feature.
ip gdp irdpThis command has no arguments or keywords.
Disabled
Interface configuration
IP routing must be disabled before you can configure this feature.
The following example configures router discovery using IRDP on the Ethernet 0 interface:
interface ethernet 0
ip gdp irdp
Use the ip gdp rip interface configuration command to configure the router discovery feature using the Routing Information Protocol (RIP). Use the no ip gdp rip command to disable this feature.
ip gdp ripThis command has no arguments or keywords.
Disabled
Interface configuration
IP routing must be disabled before you can configure this feature.
The following example configures router discovery using RIP on the Ethernet 1 interface:
interface ethernet 1
ip gdp rip
Use the ip helper-address interface configuration command to tell the router to forward User Datagram Protocol (UDP) broadcasts, including BootP, received on an interface. Use the no ip helper-address command to disable the forwarding of broadcast packets to specific addresses.
ip helper-address address| address | Destination broadcast or host address to be used when forwarding UDP broadcasts. You can have more than one helper address per interface. |
Disabled
Interface configuration
Combined with the ip forward-protocol global configuration command, the ip helper-address command allows you to control which broadcast packets and which protocols are forwarded.
The following example defines an address that acts as a helper address:
interface ethernet 1
ip helper-address 121.24.43.2
ip forward-protocol
Use the ip host global configuration command to define a static host name-to-address mapping in the host cache. Use the no ip host command to remove the name-to-address mapping.
ip host name [tcp-port-number] address1 [address2...address8]| name | Name of the host. The first character can be either a letter or a number, but if you use a number, the operations you can perform are limited. |
| tcp-port-number | (Optional) TCP port number--Telnet by default (port 23). Used as the default TCP port to connect to when using the defined host name in conjunction with an EXEC connect or telnet command. |
| address | Associated IP address. Up to eight addresses can be bound to a host name. |
Disabled
Global configuration
The first character can be either a letter or a number, but if you use a number, the operations you can perform (such as ping) are limited.
The following example uses the ip host command to define two static mappings:
ip host croff 192.31.7.18
ip host bisso-gw 10.2.0.2 192.31.7.33
Use the ip hp-host global configuration command to enter the host name of an HP host to be used for HP Probe Proxy service into the host table. Use the no ip hp-host command with the appropriate arguments to remove the host name.
ip hp-host hostname ip-address| hostname | Name of the host |
| ip-address | IP address of the host |
No host names are defined.
Global configuration
To use the HP Proxy service, you must first enter the host name of the HP host into the host table using this command.
The following example specifies an HP host's name and address, and then enables Probe Proxy:
ip hp-host BCWjo 131.108.1.27
interface ethernet 0
ip probe proxy
ip probe proxy
Use the ip mask-reply interface configuration command to tell the router to respond to Internet Control Message Protocol (ICMP) mask requests by sending ICMP Mask Reply messages. Use the no ip mask-reply command to disable this function.
ip mask-replyThis command has no arguments or keywords.
Disabled
Interface configuration
The following example enables the sending of ICMP Mask Reply messages on interface Ethernet 0:
interface ethernet 0
ip address 131.108.1.0 255.255.255.0
ip mask-reply
Use the ip mtu interface configuration command to set the maximum transmission unit (MTU) size of IP packets sent on an interface. Use the no ip mtu command to restore the default.
ip mtu bytes| bytes | IP MTU in bytes |
Minimum is 128 bytes; maximum depends on interface medium type.
Interface configuration
If an IP packet exceeds the MTU set for the router's interface, the router will fragment it.
All devices on a physical medium must have the same protocol MTU in order to operate.
The following example sets the maximum IP packet size for the first serial interface to 300 bytes:
interface serial 0
ip mtu 300
A dagger (+) indicates that the command is documented in another chapter.
mtu +
Use the ip name-server global configuration command to specify the address of one or more name servers to use for name and address resolution. Use the no ip name-server command to remove the addresses specified and restore the default.
ip name-server server-address1 [[server-address2]... server-address6]| server-address1...6 | IP addresses of up to six name servers |
No name server addresses are specified.
Global configuration
The following example specifies host 131.108.1.111 as the primary name server and host 131.108.1.2 as the secondary server:
ip name-server 131.108.1.111 131.108.1.2
This command will be reflected in the configuration file as follows:
ip name-server 131.108.1.111
ip name-server 131.108.1.2
ip domain-lookup
ip domain-name
Use the ip probe proxy interface configuration command to enable the HP Probe Proxy support that allows a router to respond to HP Probe Proxy Name requests. Use the no ip probe proxy command to disable HP Probe Proxy.
ip probe proxyThis command has no arguments or keywords.
Disabled
Interface configuration
HP Probe Proxy Name requests are typically used at sites that have HP equipment and are already using HP Probe.
To use the HP Proxy service, you must first enter the host name of the HP host into the host table using the ip hp-host global configuration command.
The following example specifies an HP host's name and address, and then enables Probe Proxy:
ip hp-host BCWjo 131.108.1.27
interface ethernet 0
ip probe proxy
ip hp-host
Use the ip proxy-arp interface configuration command to enable proxy ARP on an interface. Use the no ip proxy-arp command to disable proxy ARP on the interface.
ip proxy-arpThis command has no arguments or keywords.
Enabled
Interface configuration
The following example enables proxy ARP on interface Ethernet 0:
interface ethernet 0
ip proxy-arp
Use the ip redirects interface configuration command to enable the sending of redirect messages if the router is forced to resend a packet through the same interface on which it was received. Use the no ip redirects command to disable the sending of redirect messages.
ip redirectsThis command has no arguments or keywords.
Enabled
Interface configuration
The following example enables the sending of IP redirects on interface Ethernet 0:
interface ethernet 0
ip redirects
show ip redirects
Use the ip route-cache interface configuration command to control the use of a high-speed switching cache for IP routing as well as the use of autonomous switching. Use the no ip route-cache command to disable fast switching and autonomous switching.
ip route-cache [cbus] IP autonomous switching is disabled.
Fast switching varies by interface and media.
SSE switching of IP is disabled.
Interface configuration
Using the route cache is often called fast switching. The route cache allows outgoing packets to be load-balanced on a per-destination basis.
The ip route-cache command with not additional keywords, enables fast switching.
Our routers generally offer better packet transfer performance when fast switching is enabled, with one exception. On networks using slow serial links (64K and below), disabling fast switching to enable the per-packet load sharing is usually the best choice.
Autonomous switching gives a router faster packet processing by allowing the ciscoBus to switch packets independently without interrupting the system processor. It works only in Cisco 7000 series or AGS+ systems with high-speed network controller cards, and with a switch processor or ciscoBus controller card running microcode Version 1.4 or later.
You can enable IP fast switching when the input and output interfaces are the same interface, using the ip route-cache same-interface command. This normally is not recommended, though it is useful when you have partially meshed media, such as Frame Relay. You could use this feature on other interfaces, although it is not recommended because it would interfere with redirection.
SSE switching gives a router even faster packet processing than the is provided by the other ip route-cache commands by allowing the SSE to switch packets without interrupting the system processor. SSE switching is supported only in Cisco 7000 systems with an SSP board. Fast switching must be active to enable SSE switching. SSE switching requires that fast switching be enabled.
The following example enables both fast switching and autonomous switching:
ip route-cache cbus
The following example disables both fast switching and autonomous switching:
no ip route-cache
The following example turns off autonomous switching only:
no ip route-cache cbus
The following example returns the system to its defaults (fast switching enabled; autonomous switching disabled):
ip route-cache
ip cache-invalidate-delay
show ip cache
Use the ip routing global configuration command to enable IP routing. Use the no ip routing command to disable IP routing for the router.
ip routingThis command has no arguments or keywords.
Enabled
Global configuration
If the system is running bridging software, the no ip routing command turns off IP routing when setting up a system to bridge (as opposed to route) IP packets.
The following example shows how to enable IP routing:
ip routing
Use the ip security add interface configuration command to add a basic security option to all outgoing packets. Use the no ip security add command to disable the adding of a basic security option to all outgoing packets.
ip security addThis command has no arguments or keywords.
Disabled, when the security level of the interface is "Unclassified Genser" (or unconfigured). Otherwise, the default is enabled.
Interface configuration
If an outgoing packet does not have a security option present, this interface configuration command will add one as the first IP option. The security label added to the option field is the label that was computed for this packet when it first entered the router. Because this action is performed after all the security tests have been passed, this label will either be the same as or will fall within the range of the interface.
The following example adds a basic security option to each packet leaving interface Ethernet 0:
interface ethernet 0
ip security add
ip security dedicated
ip security extended-allowed
ip security first
ip security ignore-authorities
ip security implicit-labelling
ip security multilevel
ip security reserved-allowed
ip security strip
Use the ip security aeso command to attach Auxiliary Extended Security Option (AESOs) to an interface. Use the no ip security aeso command to disable AESO on an interface.
ip security aeso source compartment-bits| source | ESO source. An integer from 0 through 255. |
| compartment-bits | Compartment bits in hex. |
Disabled
Interface configuration
Compartment bits are specified only if this AESO is to be inserted in a packet. On every incoming packet at this level on this interface, these AESOs should be present.
Beyond being recognized, no further processing of AESO information is performed. AESO contents are not checked and are assumed to be valid if the source is listed in the configurable AESO table.
Configuring any per-interface extended IP security option (IPSO) information automatically enables ip security extended-allowed (disabled by default).
In the following example, the extended security option source is defined as 5 and the compartments bits are set to 5.
interface ethernet 0
ip security aeso 5 5
ip security eso-info
ip security eso-min
ip security eso-max
ip security extended allowed
Use the ip security dedicated interface configuration command to set the requested level of classification and authority on the interface. Use the no ip security dedicated command to reset the interface to the default classification and authorities.
ip security dedicated level authority [authority...]| level | Degree of sensitivity of information. The level keywords are listed in Table 16-1. |
| authority | Organization that defines the set of security levels that will be used in a network. The authority keywords are listed in Table 16-2. |
Disabled
Interface configuration
All traffic entering the system on this interface must have a security option that exactly matches this label. Any traffic leaving via this interface will have this label attached to it.
The following definitions apply to the descriptions of the IP security options (IPSO) in this section:
| Level Keyword | Bit Pattern |
|---|---|
| Reserved4 | 0000 0001 |
| TopSecret | 0011 1101 |
| Secret | 0101 1010 |
| Confidential | 1001 0110 |
| Reserved3 | 0110 0110 |
| Reserved2 | 1100 1100 |
| Unclassified | 1010 1011 |
| Reserved1 | 1111 0001 |
| Authority Keyword | Bit Pattern |
|---|---|
| Genser | 1000 0000 |
| Siop-Esi | 0100 0000 |
| DIA | 0010 0000 |
| NSA | 0001 0000 |
| DOE | 0000 1000 |
The following example sets a confidential level with Genser authority:
ip security dedicated confidential Genser
ip security add
ip security extended-allowed
ip security first
ip security ignore-authorities
ip security implicit-labelling
ip security multilevel
ip security reserved-allowed
ip security strip
Use the ip security eso-info global configuration command to configure system-wide defaults for extended IP security option (IPSO) information. Use the no form of this command to revert to default settings.
ip security eso-info source compartment-size default-bit| source | Hexadecimal or decimal value representing the extended IPSO source. An integer from 0 through 255. |
| compartment-size | The maximum number of bytes of compartment information allowed for a particular extended IPSO source. An |
| default-bit | Default bit value for any unsent compartment bits. |
Disabled
Global configuration
This command configures Extended Security Option (ESO) information, including Auxiliary Extended Security Option (AESO). Transmitted compartment info is padded to the size specified by the compartment-size argument.
In the following example, system-wide defaults for source, compartment size, and the default bit value are set:
ip security eso-info 100 5 1
ip security eso-max
ip security eso-min
Use the ip security eso-min interface configuration command to configure the minimum sensitivity for an interface. Use the no form of this command to revert to the default.
ip security eso-min source compartment-bits| source | Extended Security Option (ESO) source. An integer from 1 through 255. |
| compartment-bits | Compartment bits in hex. |
Disabled
Interface configuration
This command is used to specify the minimum sensitivity level for a particular interface. Before the per-interface compartment information for a particular Network Level Extended Security Option (NLESO) source can be configured, the ip security eso-info global configuration command must be used to specify the default information.
On every incoming packet on this interface, these extended security options should be resent at the minimum level and should match the configured compartment bits. Every outgoing packet must have these ESOs.
On every packet transmitted or received on this interface, any NLESO sources present in the IP header should be bounded by the minimum sensitivity level and bounded by the maximum sensitivity level configured for the interface.
When transmitting locally generated traffic out this interface, or adding security information (with the ip security add command), the maximum compartment bit information can be used to construct the NLESO sources placed in the IP header.
A maximum of 16 NLESO sources can be configured per interface. Due to IP header length restrictions, a maximum of 9 of these NLESO sources appear in the IP header of a packet.
In the following example, the specified ESO source is 5 and the compartment bits are specified as 5.
interface ethernet 0
ip security eso-min 5 5
ip security eso-max
ip security eso-info
Use the ip security eso-max interface configuration command to specify the maximum sensitivity level for an interface. Use the no form of this command to revert to the default.
ip security eso-max source compartment-bits| source | Extended Security Option (ESO) source. An integer from 1 through 255. |
| compartment-bits | Compartment bits in hex. |
Disabled
Interface configuration
This command is used to specify the minimum sensitivity level for a particular interface. Before the per interface compartment information for a particular Network Level Extended Security Option (NLESO) source can be configured, the ip security eso-info global configuration command must be used to specify the default information.
On every incoming packet on the interface, these extended security options should be resent at the minimum level and should match the configured compartment bits. Every outgoing packet must have these ESOs.
On every packet transmitted or received on this interface, any NLESO sources present in the IP header should be bounded by the minimum sensitivity level and bounded by the maximum sensitivity level configured for the interface.
When transmitting locally generated traffic out this interface, or adding security information (with the ip security add command), the maximum compartment bit information can be used to construct the NLESO sources placed in the IP header.
A maximum of 16 NLESO sources can be configured per interface. Due to IP header length restrictions, a maximum of 9 of these NLESO sources appear in the IP header of a packet.
In the following example, the specified ESO source is 240 and the compartment bits are specified as 500.
interface ethernet 0
ip security eso-max 240 500
ip security eso-min
ip security eso-info
Use the ip security extended-allowed interface configuration command to accept packets on an interface that has an extended security option present. Packets containing extended security options are rejected. Use the no ip security extended-allowed command to restore the default.
ip security extended-allowedThis command has no arguments or keywords.
Disabled
Interface configuration
The following example allows interface Ethernet 0 to accept packets that have an extended security option present:
interface ethernet 0
ip security extended-allowed
ip security add
ip security dedicated
ip security first
ip security ignore-authorities
ip security implicit-labelling
ip security multilevel
ip security reserved-allowed
ip security strip
Use the ip security first interface configuration command to prioritize the presence of security options on a packet. Use the no ip security first command to turn off this function.
ip security firstThis command has no arguments or keywords.
Disabled
Interface configuration
If a basic security option is present on an outgoing packet, but it is not the first IP option, then the packet is moved to the front of the options field when this interface configuration command is used.
The following example ensures that, if a basic security option is present in the options field of a packet exiting interface Ethernet 0, the packet is moved to the front of the options field.
interface ethernet 0
ip security first
ip security add
ip security dedicated
ip security extended-allowed
ip security ignore-authorities
ip security implicit-labelling
ip security multilevel
ip security reserved-allowed
ip security strip
Use the ip security ignore-authorities interface configuration command to cause the router to ignore the authorities field of all incoming packets. Use the no ip security ignore-authorities command to turn off this function.
ip security ignore-authoritiesThis command has no arguments or keywords.
Disabled
Interface configuration
When the packet's authority field is ignored, the value used in place of this field is the authority value declared for the specified interface. IP security ignore-authorities can only be configured on interfaces with dedicated security levels.
The following example causes interface Ethernet 0 to ignore the authorities field on all incoming packets:
interface ethernet 0
ip security ignore-authorities
ip security add
ip security dedicated
ip security extended-allowed
ip security first
ip security implicit-labelling
ip security multilevel
ip security reserved-allowed
ip security strip
Use the ip security implicit-labelling interface configuration command to force the router to accept packets on the interface, even if they do not include a security option. Use the no ip security implicit-labelling command to disable this function.
ip security implicit-labelling [level authority [authority...]]| level | (Optional) Degree of sensitivity of information. If your interface has multilevel security set, you must specify this argument. The level keywords are listed in Table 16-1 (see the ip security dedicated interface configuration command). |
| authority | (Optional) Organization that defines the set of security levels that will be used in a network. If your interface has multilevel security set, you must specify this argument. You can specify more than one. The authority keywords are listed in Table 16-2 (see the ip security dedicated interface configuration command). |
Enabled, when the security level of the interface is "Unclassified Genser" (or unconfigured). Otherwise, the default is disabled.
Interface configuration
If your interface has multilevel security set, you must use the expanded form of the command (with the optional arguments as noted in brackets) because the arguments are used to specify the precise level and authority to use when labeling the packet. If your interface has dedicated security set, the additional arguments are ignored.
In the following example, an interface is set for security and will accept unlabeled packets:
ip security dedicated confidential genser
ip security implicit-labelling
ip security add
ip security dedicated
ip security extended-allowed
ip security first
ip security ignore-authorities
ip security multilevel
ip security reserved-allowed
ip security strip
Use the ip security multilevel interface configuration command to set the interface to the requested range of classifications and authorities. All traffic entering or leaving the system must have a security option that falls within this range.
ip security multilevel level1 [authority1...] to level2 authority2 [authority2...]| level1 | Degree of sensitivity of information. The classification level of incoming packets must be equal to or greater than this value for processing to occur. The level keywords are found in Table 16-1 (see the ip security dedicated command). |
| authority1 | (Optional) Organization that defines the set of security levels that will be used in a network. The authority bits must be a superset of this value. The authority keywords are listed in Table 16-2 (see the ip security dedicated command). |
| to | Separates the range of classifications and authorities. |
| level2 | Degree of sensitivity of information. The classification level of incoming packets must be equal to or less than this value for processing to occur. The level keywords are found in Table 16-1 (see the ip security dedicated command). |
| authority2 | Organization that defines the set of security levels that will be used in a network. The authority bits must be a proper subset of this value. The authority keywords are listed in Table 16-2 (see the ip security dedicated command). |
Disabled
Interface configuration
Being within range requires that the following two conditions be met:
The following example specifies levels Unclassified to Secret and NSA authority:
ip security multilevel unclassified to secret nsa
ip security add
ip security dedicated
ip security extended-allowed
ip security first
ip security ignore-authorities
ip security implicit-labelling
ip security reserved-allowed
ip security strip
Use the ip security reserved-allowed interface configuration command to treat as valid any packets that have Reserved1 through Reserved4 security levels. Use the no ip security reserved-allowed command to disable this feature.
ip security reserved-allowedThis command has no arguments or keywords.
Disabled
Interface configuration
When you set multilevel security on an interface, and indicate, for example, that the highest range allowed is Confidential, and the lowest is Unclassified, the router neither allows nor operates on packets that have security levels of Reserved3 and Reserved2 because they are undefined.
If you use the IP Security Option (IPSO) to block transmission out of unclassified interfaces, and you use one of the Reserved security levels, you must enable this feature to preserve network security.
The following example allows a security level of Reserved through interface Ethernet 0:
interface ethernet 0
ip security reserved-allowed
ip security add
ip security dedicated
ip security extended-allowed
ip security first
ip security ignore-authorities
ip security implicit-labelling
ip security multilevel
ip security strip
Use the ip security strip interface configuration command to remove any basic security option on outgoing packets on an interface. Use the no ip security strip command to disable this function.
ip security stripThis command has no arguments or keywords.
Disabled
Interface configuration
This procedure is performed after all security tests in the router have been passed. This command is not allowed for multilevel interfaces.
The following example removes any basic security options on outgoing packets on interface Ethernet 0:
interface ethernet 0
ip security strip
ip security add
ip security dedicated
ip security extended-allowed
ip security first
ip security ignore-authorities
ip security implicit-labelling
ip security multilevel
ip security reserved-allowed
Use the ip source-route global configuration command to allow the router to handle IP datagrams with source routing header options. Use the no ip source-route command to cause the system to discard any IP datagram containing a source-route option.
ip source-routeThis command has no arguments or keywords.
Enabled
Global configuration
The following example enables the handling of IP datagrams with source routing header options:
ip source-route
ping
Use the ip subnet-zero global configuration command to enable use of subnet zero for interface addresses and routing updates. Hence, it provides the ability to configure and route to subnet-zero subnets. Use the no ip subnet-zero command to restore the default.
ip subnet-zeroThis command has no arguments or keywords.
Disabled
Global configuration
Subnetting with a subnet address of zero is discouraged because of the confusion inherent in having a network and a subnet with indistinguishable addresses.
In the following example, subnet-zero is enabled for the router:
ip subnet-zero
Use the ip tcp compression-connections interface configuration command to specify the total number of header compression connections that can exist on an interface. Use the no ip tcp compression-connections command to restore the default.
ip tcp compression-connections number| number | Number of connections the cache will support; number can vary between 3 and 256, inclusive. |
16 connections
Interface configuration
You should configure one connection for each TCP connection through the specified interface.
Each connection sets up a compression cache entry, so you are in effect specifying the maximum number of cache entries and the size of the cache. Too few cache entries for the specified interface can lead to degraded performance, while too many cache entries can lead to wasted memory.
In the following example, the first serial interface is set for header compression with a maximum of ten cache entries:
interface serial 0
ip tcp header-compression
ip tcp compression-connections 10
ip tcp header-compression
show ip tcp header-compression
Use the ip tcp header-compression interface configuration command to enable TCP header compression. Use the no ip tcp header-compression command to disable compression.
ip tcp header-compression [passive]| passive | (Optional) Outgoing TCP packets are compressed only if incoming TCP packets on the same interface are compressed. If you do not specify the passive keyword, the router compresses all traffic. |
Disabled
Interface configuration
You can compress the headers of your TCP/IP packets in order to reduce the size of your packets. TCP header compression is supported on serial lines using HDLC or PPP encapsulation. You must enable compression on both ends of a serial connection. RFC 1144 specifies the compression process. Compressing the TCP header can speed up Telnet connections dramatically. In general, TCP header compression is advantageous when your traffic consists of many small packets, not for traffic that consists of large packets. Transaction processing (usually using terminals) tends to use small packets while file transfers use large packets. This feature only compresses the TCP header, so it has no effect on UDP packets or other protocol headers.
When compression is enabled, fast switching is disabled. This means that fast interfaces like T1 can overload the router. Consider your network's traffic characteristics before using this command.
In the following example, the first serial interface is set for header compression with a maximum of ten cache entries:
interface serial 0
ip tcp header-compression
ip tcp compression-connections 10
ip tcp compression-connections
show ip tcp header-compression
To enable Path MTU Discovery for all new TCP connections from the router, use the ip tcp path-mtu-discovery interface configuration command. To disable the feature, use the no form of this command.
ip tcp path-mtu-discoveryThis command has no arguments or keywords.
Disabled
Interface configuration
Path MTU Discovery is a method for maximizing the use of available bandwidth in the network between the end points of a TCP connection. It is described in RFC 1191. Existing connections are not affected when this feature is turned on or off.
Customers using TCP connections to move bulk data between systems on distinct subnets would benefit most by enabling this feature. This might include customers using RSRB with TCP encapsulation, STUN, X.25 Remote Switching (also known as XOT or X.25 over TCP), and some protocol translation configurations.
In the following example, Path MTU Discovery is enabled:
ip tcp path-mtu-discovery
Use the ip tcp synwait-time global configuration command to set a specified period of time the router will wait to attempt to establish a TCP connection before it times out. The no ip tcp synwait-time command restores the default.
ip tcp synwait-time seconds| seconds | Number of seconds the router waits to attempt to establish a TCP connection. Use any value between 5 and 300 seconds. |
30 seconds
Global configuration
In previous versions of router software, the system would wait a fixed 30 seconds when attempting to establish a TCP connection. If your network contains Public Switched Telephone Network Dial on Demand Routing (PSTN DDR), it is possible that the call setup time will exceed 30 seconds. This amount of time is not sufficient in networks that have dial-up asynchronous connections because it will affect your ability to Telnet over the link (from the router) if the link must be brought up. If you have this type of network, you might want to set this value to the UNIX value of 75.
Because this is a host parameter, it does not pertain to traffic going through the router, just for traffic originated at the router. Because UNIX has a fixed 75-second timeout, hosts are unlikely to see this problem.
The following example configures the router to continue attempting to establish a TCP connection for 180 seconds:
ip tcp synwait-time 180
Use the ip unnumbered interface configuration command to enable IP processing on a serial interface without assigning an explicit IP address to the interface. Use the no ip unnumbered command to disable the IP processing on the interface.
ip unnumbered interface-name| interface-name | Name of another interface on which the router has an assigned IP address. This interface-name cannot be another unnumbered interface. |
Disabled
Interface configuration
Whenever the unnumbered interface generates a packet (for example, for a routing update), it uses the address of the specified interface as the source address of the IP packet. It also uses the address of the specified interface in determining which routing processes are sending updates over the unnumbered interface. Restrictions include the following:
The interface you specify by the interface-name argument must be enabled (listed as "up" in the show interfaces command display).
If you are configuring IS-IS across a serial line, you should configure the serial interfaces as unnumbered. This allows you to conform with RFC 1195, which states that IP addresses are not required on each interface.
In the following example, the first serial interface is given Ethernet 0's address:
interface ethernet 0
ip address 131.108.6.6 255.255.255.0
interface serial 0
ip unnumbered ethernet 0
Use the ip unreachables interface configuration command to enable the generation of ICMP Unreachable messages on a specified interface. Use the no ip unreachables command to disable this function.
ip unreachablesThis command has no arguments or keywords.
Enabled
Interface configuration
If the router receives a nonbroadcast packet destined for itself that uses a protocol it does not recognize, it sends an ICMP Protocol Unreachable message to the source.
If the router receives a datagram that it cannot deliver to its ultimate destination because it knows of no route to the destination address, it replies to the originator of that datagram with an ICMP Host Unreachable message.
This command affects all kinds of ICMP unreachable messages.
The following example enables the generation of ICMP Unreachable messages, as appropriate, on an interface:
interface ethernet 0
ip unreachables
Use the ping (IP packet internet groper function) user EXEC command to send ICMP Echo messages to check host reachability and network connectivity. If the router receives an ICMP Echo message, it sends an ICMP Echo Reply message to the source of the ICMP Echo message.
ping [protocol] {host | address}| protocol | (Optional) Protocol keyword. IP is the default. |
| host | Host name of system to ping. |
| address | IP address of system to ping. |
EXEC
The user ping feature provides a basic ping facility for IP users who do not have system privileges. This feature allows the router to perform the simple default ping functionality for the IP protocol. Only the nonverbose form of the ping command is supported for user pings.
If the system cannot map an address for a host name, it will return an "%Unrecognized host or address" error message.
To abort a ping session, type the escape sequence (by default, Ctrl-^ X, which is done by simultaneously pressing the Ctrl, Shift, and 6 keys, letting go, then pressing the X key).
Table 16-3 describes the test characters that the ping facility sends.
| Char | Description |
|---|---|
| ! | Each exclamation point indicates receipt of a reply. |
| . | Each period indicates the network server timed out while waiting for a reply. |
| U | Destination unreachable. |
| N | Network unreachable. |
| P | Protocol unreachable. |
| Q | Source quench. |
| M | Could not fragment. |
| ? | Unknown packet type. |
The following display shows sample ping output when you ping a host named fred:
Router> ping fred
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.31.7.27, timeout is 2 seconds:
!!!!!
Success rate is 100 percent, round-trip min/avg/max = 1/3/4 ms
The following display shows sample ping output when you ping the broadcast address of 255.255.255.255:
Router> ping 255.255.255.255
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 255.255.255.255, timeout is 2 seconds:
Reply to request 0 from 160.89.48.15 (4 ms)
Reply to request 0 from 160.89.48.10 (4 ms)
Reply to request 0 from 160.89.48.19 (4 ms)
Reply to request 0 from 160.89.49.15 (4 ms)
Reply to request 1 from 160.89.48.15 (4 ms)
Reply to request 1 from 160.89.48.10 (4 ms)
Reply to request 1 from 160.89.48.19 (4 ms)
Reply to request 1 from 160.89.49.15 (4 ms)
Reply to request 2 from 160.89.48.15 (4 ms)
Reply to request 2 from 160.89.48.10 (4 ms)
Reply to request 2 from 160.89.48.19 (4 ms)
Reply to request 2 from 160.89.49.15 (4 ms)
Reply to request 3 from 160.89.48.15 (4 ms)
Reply to request 3 from 160.89.48.10 (4 ms)
Reply to request 3 from 160.89.48.19 (4 ms)
Reply to request 3 from 160.89.49.15 (4 ms)
Reply to request 4 from 160.89.48.15 (4 ms)
Reply to request 4 from 160.89.48.10 (4 ms)
Reply to request 4 from 160.89.48.19 (4 ms)
Reply to request 4 from 160.89.49.15 (4 ms)
ping (privileged)
Use the ping (IP packet internet groper function) privileged EXEC command to send ICMP Echo messages to check host reachability and network connectivity. If the router receives an ICMP Echo message, it sends an ICMP Echo Reply message to the source of the ICMP Echo message.
ping [protocol] {host | address}| protocol | (Optional) Protocol keyword. IP is the default. |
| host | Host name of system to ping. |
| address | IP address of system to ping. |
Privileged EXEC
You can use the IP ping command to diagnose serial line problems. By placing the local or remote CSU/DSU into loopback mode and pinging your own interface, you can isolate the problem to the router or leased line.
Multicast and broadcast pings are fully supported. When you ping the broadcast address of 255.255.255.255, the system will send out pings and print a list of all stations responding. You can also ping a local network to get a list of all systems that respond, as in the following example, where 128.111.3 is a local network:
ping 128.111.3.255
As a side-effect, you also can get a list of all multicast-capable hosts that are connected directly to the router from which you are pinging, as in the following example:
ping 224.0.0.1
To abort a ping session, type the escape sequence (by default, Ctrl-^ X, which is done by simultaneously pressing the Ctrl, Shift, and 6 keys, letting go, then pressing the X key).
Table 16-4 describes the test characters that the ping facility sends.
| Char | Description |
|---|---|
| ! | Each exclamation point indicates receipt of a reply. |
| . | Each period indicates the network server timed out while waiting for a reply. |
| U | Destination unreachable. |
| N | Network unreachable. |
| P | Protocol unreachable. |
| Q | Source quench. |
| M | Could not fragment. |
| ? | Unknown packet type. |
You can use the extended command mode of the ping command to specify the supported Internet header options, as shown in the following sample display.
To enter ping extended command mode, enter yes at the extended commands prompt of the ping command. The following display shows a sample ping extended command sequence.
Router# ping
Protocol [ip]:
Target IP address: 192.31.7.27
Repeat count [5]:
Datagram size [100]:
Timeout in seconds [2]:
Extended commands [n]: y
Source address: 131.108.1.1
Type of service [0]:
Set DF bit in IP header? [no]:
Data pattern [0xABCD]:
Loose, Strict, Record, Timestamp, Verbose[none]:
Sweep range of sizes [n]:
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.31.7.27, timeout is 2 seconds:
!!!!!
Success rate is 100 percent, round-trip min/avg/max = 1/3/4 ms
Table 16-5 describes significant fields shown in the display.
| Field | Description |
|---|---|
| Protocol [ip]: | Default is IP. |
| Target IP address: | Prompts for the IP address or host name of the destination node you plan to ping. |
| Repeat count [5]: | Number of ping packets that will be sent to the destination address. Default: 5. |
| Datagram size [100]: | Size of the ping packet (in bytes). Default: 100 bytes. |
| Timeout in seconds [2]: | Timeout interval. Default: 2 (seconds). |
| Extended commands [n]: | Specifies whether or not a series of additional commands appears. Many of the following displays and tables show and describe these commands. Default: no. |
| Source address: | IP address that appears in the ping packet as the source address. |
| Type of service [0]: | Internet service quality selection. See RFC 791 for more information. Default: 0. |
| Set DF bit in IP header? | Don't Fragment. Specifies that if the packet encounters a node in its path that is configured for a smaller MTU than the packet's MTU, that the packet is to be dropped and an error message is to be sent to the router at the packet's source address. If performance problems are encountered on the network, a node configured for a small MTU could be a contributing factor. This feature can be used to determine the smallest MTU in the path. Default: no. |
| Data pattern [0xABCD]: | Sets 16-bit hexadecimal data pattern. Default: 0xABCD. Varying the data pattern in this field (to all ones or all zeros for example) can be useful when debugging data sensitivity problems on CSU/DSUs, or detecting cable-related problems such as cross talk. |
| Loose, Strict, Record, Timestamp, Verbose [none]: | Supported Internet header options. The router examines the header options to every packet that passes through it. If it finds a packet with an invalid option, the router sends an ICMP Parameter Problem message to the source of the packet and discards the packet. The Internet header options follow:
Default: none. For more information on these header options, see RFC 791. |
| Sweep range of sizes [n]: | Allows you to vary the sizes of the echo packets being sent. This capability is useful for determining the minimum sizes of the MTUs configured on the nodes along the path to the destination address. Packet fragmentation contributing to performance problems can then be reduced. |
| !!!!! | Each exclamation point (!) indicates receipt of a reply. A period (.) indicates the network server timed out while waiting for a reply. Other characters may appear in the ping output display, depending on the protocol type. |
| Success rate is 100 percent | Percentage of packets successfully echoed back to the router. Anything less than 80 percent is usually considered problematic. |
| round-trip min/avg/max = 1/3/4 ms | Round-trip travel time intervals for the protocol echo packets, including minimum/average/maximum (in milliseconds). |
Using the Record Route option to trace a path to a particular destination address. Be aware, however, that the trace EXEC command performs a similar function, but the latter does not have the nine-hop limitation.
The following display shows sample extended ping output when this option is specified:
Router# ping
Protocol [ip]:
Target IP address: fred
Repeat count [5]:
Datagram size [100]:
Timeout in seconds [2]:
Extended commands [n]: y
Source address:
Type of service [0]:
Set DF bit in IP header? [no]:
Data pattern [0xABCD]:
Loose, Strict, Record, Timestamp, Verbose[none]: r
Number of hops [ 9 ]:
Loose, Strict, Record, Timestamp, Verbose[RV]:
Sweep range of sizes [n]:
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 131.108.1.115, timeout is 2 seconds:
Packet has IP options: Total option bytes= 39, padded length=40
Record route: <*> 0.0.0.0 0.0.0.0 0.0.0.0 0.0.0.0
0.0.0.0 0.0.0.0 0.0.0.0 0.0.0.0 0.0.0.0
The following display is a detail of the Echo packet section:
0 in 4 ms. Received packet has options
Total option bytes= 40, padded length=40
Record route: 160.89.80.31 131.108.6.10 131.108.1.7 131.108.1.115
131.108.1.115 131.108.6.7 160.89.80.240 160.89.80.31 <*> 0.0.0.0
End of list
1 in 8 ms. Received packet has options
Total option bytes= 40, padded length=40
Record route: 160.89.80.31 131.108.6.10 131.108.1.6 131.108.1.115
131.108.1.115 131.108.6.7 160.89.80.240 160.89.80.31 <*> 0.0.0.0
End of list
2 in 4 ms. Received packet has options
Total option bytes= 40, padded length=40
Record route: 160.89.80.31 131.108.6.10 131.108.1.7 131.108.1.115
131.108.1.115 131.108.6.7 160.89.80.240 160.89.80.31 <*> 0.0.0.0
End of list
3 in 8 ms. Received packet has options
Total option bytes= 40, padded length=40
Record route: 160.89.80.31 131.108.6.10 131.108.1.6 131.108.1.115
131.108.1.115 131.108.6.7 160.89.80.240 160.89.80.31 <*> 0.0.0.0
End of list
4 in 4 ms. Received packet has options
Total option bytes= 40, padded length=40
Record route: 160.89.80.31 131.108.6.10 131.108.1.7 131.108.1.115
131.108.1.115 131.108.6.7 160.89.80.240 160.89.80.31 <*> 0.0.0.0
End of list
Success rate is 100 percent, round-trip min/avg/max = 4/5/8 ms
Router#
In this display, five ping echo packets are sent to the destination address 131.108.1.115. The echo packet detail section includes specific information about each of these echo packets.
The lines of ping output that are unique when the Record Route option is specified are described as follows.
The following line of output allows you to specify the number of hops that will be recorded in the route. Range: 1 through 9. Default: 9.
Number of hops [ 9 ]:
The following line of output indicates that IP header options have been enabled on the outgoing echo packets and shows the number of option bytes and padded bytes in the headers of these packets.
Packet has IP options: Total option bytes= 39, padded length=40
The following lines of output indicate that the fields that will contain the IP addresses of the nodes in the routes have been zeroed out in the outgoing packets.
Record route: <*> 0.0.0.0 0.0.0.0 0.0.0.0 0.0.0.0
0.0.0.0 0.0.0.0 0.0.0.0 0.0.0.0 0.0.0.0
The following lines of output display statistics for the first of the five echo packets sent. 0 is the number assigned to this packet to indicate that it is the first in the series. 4 ms indicates the round trip travel time for the packet.
0 in 4 ms. Received packet has options
Total option bytes= 40, padded length=40
Record route: 160.89.80.31 131.108.6.10 131.108.1.7 131.108.1.115
131.108.1.115 131.108.6.7 160.89.80.240 160.89.80.31 <*> 0.0.0.0
The following line of output indicates that four nodes were included in the packet's route, including the router at source address 160.89.80.31, two intermediate nodes at addresses 131.108.6.10 and 131.108.1.7, and the destination node at address 131.108.1.115. The underlined address shows where the original route differs from the return route in the line that follows this line.
Record route: 160.89.80.31 131.108.6.10 131.108.1.7 131.108.1.115
The following line of output includes the addresses of the four nodes in the return path of the echo packet. The underlined address shows where the return route differs from the original route shown in the previous line of output.
131.108.1.115 131.108.6.7 160.89.80.240 160.89.80.31 <*> 0.0.0.0
ping (user)
Use the show access-lists privileged EXEC command to display the contents of all current access lists.
show access-listsThis command has no arguments or keywords.
Privileged EXEC
The following is sample output from the show access-lists command:
Router# show access-lists
Standard IP access list 19
permit 131.108.19.0
deny 0.0.0.0, wildcard bits 255.255.255.255
Standard IP access list 49
permit 131.108.31.0, wildcard bits 0.0.0.255
permit 131.108.194.0, wildcard bits 0.0.0.255
permit 131.108.195.0, wildcard bits 0.0.0.255
permit 131.108.196.0, wildcard bits 0.0.0.255
permit 131.108.197.0, wildcard bits 0.0.0.255
Extended IP access list 101
permit tcp 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255 eq 23
Type code access list 201
permit 0x6001 0x0000
Type code access list 202
permit 0x6004 0x0000
deny 0x0000 0xFFFF
For information on how to configure access lists, refer to the "Configuring IP" chapter of the Router Products Configuration Guide.
access-list
Use the show arp privileged EXEC command to display the entries in the ARP table for the router.
show arpThis command has no arguments or keywords.
Privileged EXEC
The following is sample output from the show arp command:
Router# show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 131.108.42.112 120 0000.a710.4baf ARPA Ethernet3
AppleTalk 4028.5 29 0000.0c01.0e56 SNAP Ethernet2
Internet 131.108.42.114 105 0000.a710.859b ARPA Ethernet3
AppleTalk 4028.9 - 0000.0c02.a03c SNAP Ethernet2
Internet 131.108.42.121 42 0000.a710.68cd ARPA Ethernet3
Internet 131.108.36.9 - 0000.3080.6fd4 SNAP TokenRing0
AppleTalk 4036.9 - 0000.3080.6fd4 SNAP TokenRing0
Internet 131.108.33.9 - 0000.0c01.7bbd SNAP Fddi0
Table 16-6 describes significant fields shown in the first line of output in the display.
| Field | Description |
|---|---|
| Protocol | Indicates the type of network address this entry includes. |
| Address | Network address that is mapped to the MAC address in this entry. |
| Age (min) | Indicates the interval (in minutes) since this entry was entered in the table, rather than the interval since the entry was last used. (The timeout value is 4 hours.) |
| Hardware Addr | MAC address mapped to the network address in this entry. |
| Type | Indicates the encapsulation type the router is using for the network address in this entry. Possible values include:
|
|
Interface | Indicates the interface associated with this network address. |
Use the show dnsix privileged EXEC command to display state information and the current configuration of the DNSIX audit writing module.
show dnsixThis command has no arguments or keywords.
Privileged EXEC
The following is sample output from the show dnsix command:
Router# show dnsix
Audit Trail Enabled with Source 128.105.2.5
State: PRIMARY
Connected to 128.105.2.4
Primary 128.105.2.4
Transmit Count 1
DMDP retries 4
Authorization Redirection List:
128.105.2.4
Record count: 0
Packet Count: 0
Redirect Rcv: 0
Use the show hosts EXEC command to display the default domain name, the style of name lookup service, a list of name server hosts, and the cached list of host names and addresses.
show hostsThis command has no arguments or keywords.
EXEC
The following is sample output from the show hosts command:
Router# show hosts
Default domain is CISCO.COM
Hame/address lookup uses domain service
Hame servers are 255.255.255.255
Host Flag Age Type Address(es)
SLAG.CISCO.COM (temp, OK) 1 IP 131.108.4.10
CHAR.CISCO.COM (temp, OK) 8 IP 192.31.7.50
CHAOS.CISCO.COM (temp, OK) 8 IP 131.108.1.115
DIRT.CISCO.COM (temp, EX) 8 IP 131.108.1.111
DUSTBIN.CISCO.COM (temp, EX) 0 IP 131.108.1.27
DREGS.CISCO.COM (temp, EX) 24 IP 131.108.1.30
Table 16-7 describes significant fields shown in the display.
| Field | Description |
|---|---|
| Flag | A temporary entry is entered by a name server; the router removes the entry after 72 hours of inactivity. A perm entry is entered by a configuration command and is not timed out. Entries marked OK are believed to be valid. Entries marked ?? are considered suspect and subject to revalidation. Entries marked EX are expired. |
| Age | Indicates the number of hours since the router last referred to the cache entry. |
| Type | Identifies the type of address, for example, IP, CLNS, or X.121. If you have used the ip hp-host global configuration command, the show hosts command will display these host names as type HP-IP. |
| Address(es) | Shows the address of the host. One host may have up to eight addresses. |
clear host
Use the show ip accounting privileged EXEC command to display the active accounting or checkpointed database.
show ip accounting [checkpoint]| checkpoint | (Optional) Indicates that the checkpointed database should be displayed. |
Privileged EXEC
Following is sample output from the show ip accounting command:
Router# show ip accounting
Source Destination Packets Bytes
131.108.19.40 192.67.67.20 7 306
131.108.13.55 192.67.67.20 67 2749
131.108.2.50 192.12.33.51 17 1111
131.108.2.50 130.93.2.1 5 319
131.108.2.50 130.93.1.2 463 30991
131.108.19.40 130.93.2.1 4 262
131.108.19.40 130.93.1.2 28 2552
131.108.20.2 128.18.6.100 39 2184
131.108.13.55 130.93.1.2 35 3020
131.108.19.40 192.12.33.51 1986 95091
131.108.2.50 192.67.67.20 233 14908
131.108.13.28 192.67.67.53 390 24817
131.108.13.55 192.12.33.51 214669 9806659
131.108.13.111 128.18.6.23 27739 1126607
131.108.13.44 192.12.33.51 35412 1523980
192.31.7.21 130.93.1.2 11 824
131.108.13.28 192.12.33.2 21 1762
131.108.2.166 192.31.7.130 797 141054
131.108.3.11 192.67.67.53 4 246
192.31.7.21 192.12.33.51 15696 695635
192.31.7.24 192.67.67.20 21 916
131.108.13.111 128.18.10.1 16 1137
Table 16-8 describes significant fields shown in the display.
| Field | Description |
|---|---|
| Source | Source address of the packet. |
| Destination | Destination address of the packet. |
| Packets | Number of packets transmitted from the source address to the destination address. |
| Bytes | Number of bytes transmitted from the source address to the destination address. |
clear ip accounting
ip accounting
ip accounting-list
ip accounting-threshold
ip accounting-transits
Use the show ip aliases EXEC command to display the router's Internet addresses mapped to TCP ports (aliases) and SLIP addresses, which are treated similarly to aliases.
show ip aliasesThis command has no arguments or keywords.
EXEC
To distinguish a SLIP address from a normal alias address, the command output uses the form SLIP TTY1 for the "port" number, where 1 is the auxiliary port.
The following is sample output from the show ip aliases command:
Router# show ip aliases
IP Address Port
131.108.29.245 SLIP TTY1
The display lists the IP address and corresponding port number.
A dagger (+) indicates that the command is documented in another chapter.
show line +
Use the show ip arp EXEC command to display the Address Resolution Protocol (ARP) cache, where SLIP addresses appear as permanent ARP table entries.
show ip arpThis command has no arguments or keywords.
EXEC
ARP establishes correspondences between network addresses (an IP address, for example) and LAN hardware addresses (Ethernet addresses). A record of each correspondence is kept in a cache for a predetermined amount of time and then discarded.
The following is sample output from the show ip arp command:
Router# show ip arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 131.108.62.192 187 0800.2010.a3b6 ARPA Ethernet3
Internet 131.108.62.245 68 0800.200e.28f8 ARPA Ethernet3
Internet 131.108.1.140 139 0000.0c01.2812 ARPA Ethernet0
Internet 131.108.62.160 187 0800.200e.4dab ARPA Ethernet3
Internet 131.108.1.111 27 0800.2007.8866 ARPA Ethernet0
Internet 131.108.1.117 119 0000.0c00.f346 ARPA Ethernet0
Internet 131.108.1.115 28 0000.0c01.0509 ARPA Ethernet0
Internet 131.108.1.77 1 0800.200e.57ce ARPA Ethernet0
Internet 192.31.7.29 225 aa00.0400.0234 ARPA Ethernet2
Internet 192.31.7.17 118 2424.c01f.0711 ARPA Ethernet2
Internet 192.31.7.18 135 0000.0c01.2817 ARPA Ethernet2
Internet 192.31.7.21 119 2424.c01f.0715 ARPA Ethernet2
Internet 131.108.1.33 1 0800.2008.c52e ARPA Ethernet0
Internet 131.108.62.1 - 0000.0c00.750f ARPA Ethernet3
Internet 131.108.31.35 119 0800.2010.8c5b ARPA Ethernet7
Internet 131.108.62.7 14 0000.0c00.33ce ARPA Ethernet3
Internet 131.108.1.55 155 0800.200e.e443 ARPA Ethernet0
Table 16-9 describes significant fields shown in the display.
| Field | Description |
|---|---|
| Protocol | Protocol for network address in the Address field. |
| Address | The network address that corresponds to Hardware Addr. |
| Age (min) | Age, in minutes, of the cache entry. |
| Hardware Addr | LAN hardware address a MAC address that corresponds to network address. |
| Type | Type of encapsulation:
|
|
Interface | Interface to which this address mapping has been assigned. |
Use the show ip cache EXEC command to display the routing table cache used to fast switch Internet traffic.
show ip cacheThis command has no arguments or keywords.
EXEC
The show ip cache display shows MAC headers up to 92 bytes.
The following is sample output from the show ip cache command:
Router# show ip cache
IP routing cache version 13, entries 19/20, memory 880/1000
hash bucket overflows 0
Minimum invalidation interval 5 seconds, maximum interval 30 seconds,
quiet interval 10 seconds, threshold 5 requests
Invalidation rate 0 in last second, 5 in last 10 seconds
Cache invalidation pending for 3 seconds
Hash Destination Interface MAC Header
*6D/0 128.18.1.254 Serial0 0F000800
*81/0 131.108.1.111 Ethernet0 00000C002C83AA00040002340800
*8D/0 131.108.13.111 Ethernet0 AA0004000134AA00040002340800
99/0 128.18.10.1 Serial0 0F000800
*9B/0 128.18.10.3 Serial0 0F000800
*B0/0 128.18.5.39 Serial0 0F000800
*B6/0 128.18.3.39 Serial0 0F000800
*C0/0 131.108.12.35 Ethernet0 AA0004000134AA00040002340800
*C4/0 131.108.2.41 Ethernet0 00000C002C83AA00040002340800
*C9/0 192.31.7.17 Ethernet0 2424C01F0711AA00040002340800
*CD/0 192.31.7.21 Ethernet0 2424C01F0715AA00040002340800
*D5/0 131.108.13.55 Ethernet0 AA0004006508AA00040002340800
*DC/0 130.93.1.2 Serial0 0F000800
*DE/0 192.12.33.51 Serial0 0F000800
*DF/0 131.108.2.50 Ethernet0 AA0004000134AA00040002340800
*E7/0 131.108.3.11 Ethernet0 00000C002C83AA00040002340800
*EF/0 192.12.33.2 Serial0 0F000800
*F5/0 192.67.67.53 Serial0 0F000800
*F5/1 131.108.1.27 Ethernet0 AA0004006508AA00040002340800
*FE/0 131.108.13.28 Ethernet0 AA0004006508AA00040002340800
Table 16-10 describes significant fields shown in the display.
| Field | Description |
|---|---|
| IP routing cache version nn | Version number of this table. This number is incremented any time the table is flushed. |
| entries 19/20 | Number of valid entries/total number of entries. |
| memory 880/1000 | Number of bytes of processor memory for valid entries/total number of bytes for the entire table. |
| hash bucket overflows 0 | Number of times autonomous switching cache overflowed. |
| Minimum invalidation interval 5 seconds | Minimum time delay between cache invalidation request and actual invalidation. |
| maximum interval 30 seconds | Maximum time delay between cache invalidation request and actual invalidation. |
| quiet interval 10 seconds | Length of time during which cache must be quiet. |
| threshold 5 requests | Maximum number of requests considered quiet. |
| Invalidation rate 0 in last second | Number of cache invalidation requests in last second. |
| 5 in last 10 seconds | Number of cache invalidation requests during the last quiet interval. |
| Cache invalidation pending for 3 seconds | Length of time a pending cache invalidation request has been delayed. |
| Hash | Position in the hash table for this entry. |
| * | Designates valid cache entry. |
| Destination | Shows the destination IP address. |
| Interface | Specifies the interface type and number (serial 1, Ethernet 2, and so on). |
| MAC Header | Displays the MAC header. |
Use the show ip interface EXEC command to display the usability status of interfaces.
show ip interface [interface unit]| interface unit | (Optional) Used to display information for a particular interface. For example, e 0 specifies the first Ethernet interface; e 1 specifies the second Ethernet interface. You must specify both the interface type and unit number. |
EXEC
A router automatically enters a directly connected route in the routing table if the interface is usable. A usable interface is one through which the router can send and receive packets. If the router determines that an interface is not usable, it removes the directly connected routing entry from the routing table. Removing the entry allows the router to use dynamic routing protocols to determine backup routes to the network (if any).
If the interface can provide two-way communication, the line protocol is marked "up." If the interface hardware is usable, the interface is marked "up."
If you specify an optional interface type, you will see only information on that specific interface.
If you specify no optional parameters you will see information on all the interfaces.
The following is sample output from the show ip interface command:
Router# show ip interface
Ethernet 0 is up, line protocol is up
Internet address is 192.54.222.2, subnet mask is 255.255.255.0
Broadcast address is 192.54.222.0
Address determined by non-volatile memory
MTU is 1500 bytes
Helper address is 192.52.71.4
Secondary address 131.192.115.2, subnet mask 255.255.255.0
Outgoing access list is not set
Proxy ARP is enabled
Security level is default
Split horizon is enabled
ICMP redirects are always sent
ICMP unreachables are always sent
ICMP mask replies are never sent
IP fast switching is enabled
Gateway Discovery is disabled
IP accounting is disabled
TCP/IP header compression is disabled
Probe proxy name replies are disabled
Table 16-11 describes significant fields shown in the display.
| Field | Description |
|---|---|
| Ethernet 0 is up | If the interface hardware is usable, the interface is marked "up." For an interface to be usable, both the interface hardware and line protocol must be up. |
| line protocol is up | If the interface can provide two-way communication, the line protocol is marked "up." For an interface to be usable, both the interface hardware and line protocol must be up. |
| Broadcast address | Shows the broadcast address. |
| Helper address | Specifies a helper address, if one has been set. |
| Outgoing access list | Indicates whether or not the interface has an outgoing access list set. |
| Proxy ARP | Indicates whether Proxy ARP is enabled for the interface. |
| Security level | Specifies the IPSO security level set for this interface. |
| ICMP redirects | Specifies whether redirects will be sent on this interface. |
| ICMP unreachables | Specifies whether unreachable messages will be sent on this interface. |
| ICMP mask replies | Specifies whether mask replies will be sent on this interface. |
| IP fast switching | Specifies whether fast switching has been enabled for this interface. It is generally enabled on serial interfaces, such as this one. |
| Gateway Discovery | Specifies whether the discovery process has been enabled for this interface. It is generally disabled on serial interfaces. |
| IP accounting | Specifies whether IP accounting is enabled for this interface and what the threshold (maximum number of entries) is. |
| TCP/IP header compression | Indicates whether compression is enabled or disabled. |
| Probe proxy name | Indicates whether HP Probe proxy name replies are generated. |
Use the show ip masks EXEC command to display the masks used for network addresses and the number of subnets using each mask.
show ip masks address| address | Network address for which a mask is required |
EXEC
The show ip masks command is useful for debugging when variable-length subnet masks (VLSM) are used. It shows the number of masks associated with the network and the number of routes for each mask.
The following is sample output from the show ip masks command:
Router# show ip masks 131.108.0.0
Mask Reference count
255.255.255.255 2
255.255.255.0 3
255.255.0.0 1
Use the show ip redirects EXEC command to display the address of a default gateway (router) and the address of hosts for which a redirect has been received.
show ip redirectsThis command has no arguments or keywords.
EXEC
The following is sample output from the show ip redirects command:
Router# show ip redirects
Default gateway is 160.89.80.29
Host Gateway Last Use Total Uses Interface
131.108.1.111 160.89.80.240 0:00 9 Ethernet0
128.95.1.4 160.89.80.240 0:00 4 Ethernet0
Router#
ip redirects
Use the show ip route EXEC command to display the current state of the routing table.
show ip route [address [mask]] | [protocol]| address | (Optional) Address about which routing information should be displayed. |
| mask | (Optional) Argument for a subnet mask. |
| protocol | (Optional) Argument for a particular routing protocol, or static or connected. |
EXEC
The following is sample output from the show ip route command:
Router# show ip route 160.89.6.0
Routing entry for 160.89.6.0 (mask 255.255.255.0)
Known via "connected", distance 0, metric 0 (connected)
Tag 0
Routing Descriptor Blocks:
* directly connected, via Ethernet1
Route metric is 0, traffic share count is 1
Router#
Table 16-12 describes the significant field shown in the display.
| Field | Description |
|---|---|
| Mask | Network mask associated with the route. |
| Connected | Routing protocol name, or connected or static. |
| Distance | Administrative distance. |
| Metric | Route metric that was either configured or learned from the particular route. |
| Routing Descriptor Blocks | Up to 4: Indicates the IP address of the next hop or the interface to which the particular route is connected. |
| * | Indicates the last path used when a packet was forwarded. It pertains only to the nonfast-switched packets. However, it does not indicate what path will be used next when forwarding a nonfast-switched packet except when the paths are equal cost. |
Use the show ip route summary EXEC command to display the current state of the routing table.
show ip route summaryThis command has no arguments or keywords.
EXEC
The following is sample output from the show ip route summary command:
Router# show ip route summary
Route Source Networks Subnets Overhead Memory (bytes)
connected 0 3 126 360
static 1 2 126 360
igrp 109 747 12 31878 91080
internal 3 360
Total 751 17 32130 92160
Router#
Table 16-13 describes the fields shown in the display:
| Field | Description |
|---|---|
| Route Source | Routing protocol name, or connected, static, or internal. Internal--those routes that are in the primary routing table merely as markers to hold subnet routes. These routes are not owned by any routing protocol. There should be one of these internal routes for each subnetted network in the routing table. |
| Networks | The number of Class A, B, or C networks that are present in the routing table for each route source. |
| Subnets | The number of subnets that are present in the routing table for each route source, including host routes. |
| Overhead | Any additional memory involved in allocating the routes for the particular route source other than the memory specified under "Memory." |
| Memory | The number of bytes allocated to maintain all the routes for the particular route source. |
show ip route
Use the show ip tcp header-compression EXEC command to display statistics on TCP header compression.
show ip tcp header-compressionThis command has no arguments or keywords.
EXEC
The following is sample output from the show ip tcp header-compression command:
Router# show ip tcp header-compression
TCP/IP header compression statistics:
Interface Serial1: (passive, compressing)
Rcvd: 4060 total, 2891 compressed, 0 errors
0 dropped, 1 buffer copies, 0 buffer failures
Sent: 4284 total, 3224 compressed,
105295 bytes saved, 661973 bytes sent
1.15 efficiency improvement factor
Connect: 16 slots, 1543 long searches, 2 misses, 99% hit ratio
Five minute miss rate 0 misses/sec, 0 max misses/sec
Table 16-14 describes significant fields shown in the display.
| Field | Description |
|---|---|
| Rcvd: | |
| total | Total number of TCP packets received. |
| compressed | Total number of TCP packets compressed. |
| errors | Unknown packets. |
| dropped | Number of packets dropped due to invalid compression. |
| buffer copies | Number of packets that had to be copied into bigger buffers for decompression. |
| buffer failures | Number of packets dropped due to a lack of buffers. |
| Sent: | |
| total | Total number of TCP packets sent. |
| compressed | Total number of TCP packets compressed. |
| bytes saved | Number of bytes reduced. |
| bytes sent | Number of bytes sent. |
| efficiency improvement factor | Improvement in line efficiency because of TCP header compression. |
| Connect: | |
| number of slots | Size of the cache. |
| long searches | Indicates the number of times the software had to look to find a match. |
| misses | Indicates the number of times a match could not be made. If your output shows a large miss rate, then the number of allowable simultaneous compression connections may be too small. |
| hit ratio | Percentage of times the software found a match and was able to compress the header. |
| Five minute miss rate | Calculates the miss rate over the previous 5 minutes for a longer-term (and more accurate) look at miss rate trends. |
| max misses/sec | Maximum value of the previous field. |
ip tcp header-compression
Use the show ip traffic EXEC command to display IP protocol statistics.
show ip trafficThis command has no arguments or keywords.
EXEC
The following is sample output from the show ip traffic command:
Router# show ip traffic
IP statistics:
Rcvd: 98 total, 98 local destination
0 format errors, 0 checksum errors, 0 bad hop count
0 unknown protocol, 0 not a gateway
0 security failures, 0 bad options
Frags: 0 reassembled, 0 timeouts, 0 too big
0 fragmented, 0 couldn't fragment
Bcast: 38 received, 52 sent
Sent: 44 generated, 0 forwarded
0 encapsulation failed, 0 no route
ICMP statistics:
Rcvd: 0 checksum errors, 0 redirects, 0 unreachable, 0 echo
0 echo reply, 0 mask requests, 0 mask replies, 0 quench
0 parameter, 0 timestamp, 0 info request, 0 other
Sent: 0 redirects, 3 unreachable, 0 echo, 0 echo reply
0 mask requests, 0 mask replies, 0 quench, 0 timestamp
0 info reply, 0 time exceeded, 0 parameter problem
UDP statistics:
Rcvd: 56 total, 0 checksum errors, 55 no port
Sent: 18 total, 0 forwarded broadcasts
TCP statistics:
Rcvd: 0 total, 0 checksum errors, 0 no port
Sent: 0 total
EGP statistics:
Rcvd: 0 total, 0 format errors, 0 checksum errors, 0 no listener
Sent: 0 total
IGRP statistics:
Rcvd: 73 total, 0 checksum errors
Sent: 26 total
HELLO statistics:
Rcvd: 0 total, 0 checksum errors
Sent: 0 total
ARP statistics:
Rcvd: 20 requests, 17 replies, 0 reverse, 0 other
Sent: 0 requests, 9 replies (0 proxy), 0 reverse
Probe statistics:
Rcvd: 6 address requests, 0 address replies
0 proxy name requests, 0 other
Sent: 0 address requests, 4 address replies (0 proxy)
0 proxy name replies
Table 16-15 describes significant fields shown in the display.
| Field | Description |
|---|---|
| format errors | A gross error in the packet format, such as an impossible Internet header length. |
| bad hop count | Occurs when a packet is discarded because its time-to-live (TTL) field was decremented to zero. |
| encapsulation failed | Usually indicates that the router had no ARP request entry and therefore did not send a datagram. |
| no route | Counted when the router discards a datagram it did not know how to route. |
| proxy name reply | Counted when the router sends an ARP or Probe Reply on behalf of another host. The display shows the number of probe proxy requests that have been received and the number of responses that have been sent. |
Use the show sse summary EXEC command to display a summary of Silicon Switch Processor (SSP) statistics:
show sse summaryThis command has no arguments or keywords.
EXEC
The following is sample output from the show sse summary command:
Router# show sse summary
SSE utilization statistics
Program words Rewrite bytes Internal nodes Depth
Overhead 499 1 8
IP 0 0 0 0
IPX 0 0 0 0
SRB 0 0 0 0
CLNP 0 0 0 0
IP access lists 0 0 0
Total used 499 1 8
Total free 65037 262143
Total available 65536 262144
Free program memory
[499..65535]
Free rewrite memory
[1..262143]
Internals
75032 internal nodes allocated, 75024 freed
SSE manager process enabled, microcode enabled, 0 hangs
Longest cache computation 4ms, longest quantum 160ms at 0x53AC8
Use the show standby EXEC command to display standby protocol information.
show standbyThis command has no arguments or keywords.
EXEC
The following is sample output from the show standby command:
Router# show standby
Ethernet0
Local state is Active, priority 100, preempting
Hellotime 3 holdtime 10
Next hello sent in 0:00:00
Hot standby IP address is 198.92.72.29 configured
Active router is local
Standby router is 198.92.72.21 expires in 0:00:07
Use the standby authentication interface configuration command to configure an authentication string. Use the no standby authentication command to delete the authentication string.
standby authentication string| string | Authentication string, up to eight characters in length. |
The default string is "cisco."
Interface configuration
The authentication string is transmitted unencrypted in all Hot Standby protocol messages. The same authentication string must be configured on all routers on a cable to ensure interoperation. Authentication mismatch prevents a router from learning the designated Hot Standby IP address and the Hot Standby timer values from other routers configured with the standby protocol. Authentication mismatch does not prevent protocol events such as one router taking over as the designated router.
In the following example, "word" is configured as the authentication string required to allow Hot Standby routers to interoperate.
interface ethernet 0
standby authentication word
Use the standby group interface configuration command to specify the number of the group in which the router will participate. Use the no form of this command to use the default group.
standby group number| number | Group number. An integer between 0 and 255. |
Group number 0
Interface configuration
Each Hot Standby group operates independently of others, and selects their own active and standby routers. It is possible for multiple Hot Standby groups to be configured on one physical cable, with a unique Hot Standby MAC and IP address used for each group.
Currently, this command has no effect on Token Ring interfaces.
In the following example, the system is configured to be in standby group 0 on interface Ethernet 0, and standby group 1 on interface Ethernet 2.
interface ethernet 0
standby ip
interface ethernet 2
standby ip
standby group 1
Use the standby ip interface configuration command to activate the Hot Standby protocol on the configured interface. Use the no standby ip command to disable the standby function on an interface.
standby ip [ip-address]| ip-address | (Optional) Interface Hot Standby IP address |
Disabled
Interface configuration
The standby ip command activates the hot standby protocol on the configured interface. If an IP address is specified, that address is used as the designated address for the Hot Standby group. If no IP address is specified, the designated address is learned through the standby function. For the standby protocol to elect a designated router, at least one router on the cable must have been configured with, or learned, the designated address. Configuring the designated address on the active router always overrides a designated address that is currently in use.
When the standby ip command is enabled on an interface, the handling of proxy ARP requests is changed (unless proxy ARP was disabled). If the interface's Hot Standby state is active, proxy ARP requests are anwered using the Hot Standby group's MAC address. If the interface is in a different state, proxy ARP responses are suppressed.
In the following example, the Hot Standby protocol is enabled on interface Ethernet 0. The IP address used by the Hot Standby group will be learned using the Hot Standby protocol.
interface ethernet 0
standby ip
Use the standby preempt interface configuration command to indicate that, if the local router is configured with a priority higher than the current designated router, the local router should attempt to assume control as the designated router. Use the no standby preempt command to cause the local router to only assume control as the designated router if it receives information indicating that there is no router currently in the active state (acting as the designated router).
standby preemptThis command has no arguments or keywords.
Disabled
Interface configuration
In the following example, interface Ethernet 0 is configured to preempt the current leader if the interface has been configured with a higher priority:
interface ethernet 0
standby preempt
Use the standby priority interface configuration command to prioritize a potential Hot Standby router. Use the no form of this command to restore the priority to the default.
standby priority number| number | Priority value. An integer from 0 through 255. |
Priority of 100
Interface configuration
The assigned priority is used to help select the active and standby routers. Assuming preemption is enabled, the router with the highest priority becomes the designated router. In case of ties, the primary IP addresses are compared, and the higher IP address has priority.
In the following example, interface Ethernet 0 is assigned with priority 150:
interface ethernet 0
standby priority 150
Use the standby timers interface configuration command to configure the time between hellos and the time before other routers declare the active or standby router to be down. Use the no standby timers command to restore the timers to their default values.
standby timers hellotime holdtime| hellotime | Hello interval in seconds. An integer between 1 and 255. |
| holdtime | Time in seconds before the active or standby router is declared to be down. An integer between 1 and 255. |
1 second for hellotime, and 3 seconds for holdtime.
Interface configuration
The standby timers command configures the time between standby hellos and the time before other routers declare the active or standby router to be down. Routers on which timer values are not configured can learn timer values from the active or standby router. The timers configured on the active router always override any other timer settings. All routers in a Hot Standby group should use the same timer values. Normally, holdtime is greater than or equal to 3 times hellotime (holdtime >= 3 x hellotime).
In the following example, the time between hello packets is set to 5 seconds, and the time after which a router is considered to be down is set to 15 seconds:
interface ethernet 0
standby ip
standby timers 5 15
To configure an interface so that the router's Hot Standby priority changes based on the availability of other interfaces, use the standby track interface configuration command. To remove the tracking, use the no form of this command.
standby track type number [interface-priority]| type | Interface type (combined with interface number) that will be tracked. |
| number | Interface number (combined with interface type) that will be tracked. |
| interface-priority | (Optional) Amount by which the Hot Standby priority for the router is decremented (or incremented) when the interface goes down (or comes back up). The default value is 10. |
interface-priority: 10
Interface configuration
This command ties the router's Hot Standby priority to the availability of its interfaces. It is useful for tracking interfaces that are not configured for the Hot Standby Router Protocol.
When a tracked interface goes down, the Hot Standby priority of the router decreases by 10. If an interface is not tracked, its state changes do not affect the Hot Standby priority of the router. For each interface configured for Hot Standby, you can configure a separate list of interfaces to be tracked.
The optional argument interface-priority specifies how much to decrement the router's Hot Standby priority by when a tracked interface goes down. When the tracked interface comes back up, the router's priority is incremented by the same amount.
When multiple tracked interfaces are down and interface-priority values have been configured, these configured priority decrements are cumulative. If tracked interfaces are down, but none of them were configured with priority decrements, the default decrement is 10 and it is noncumulative.
In the following example, Ethernet interface 1 tracks Ethernet interface 0 and serial interface 0. If one or both of these two interfaces go down, the Hot Standby priority of the router decreases by 10. Because the default Hot Standby priority is 100, the priority becomes 90 when one or both of the tracked interfaces go down.
interface ethernet 1
ip address 198.92.72.37 255.255.255.240
no ip redirects
standby track ethernet 0
standby track serial 0
standby preempt
standby ip 198.92.72.46
standby preempt
standby priority
Use the trace user EXEC command to discover the IP routes the router's packets will actually take when traveling to their destination.
trace ip destination| destination | Destination address or host name on the command line. The default parameters for the appropriate protocol are assumed and the tracing action begins. |
EXEC
The trace command works by taking advantage of the error messages generated by routers when a datagram exceeds its time-to-live (TTL) value.
The trace command starts by sending probe datagrams with a TTL value of one. This causes the first router to discard the probe datagram and send back an error message. The trace command sends several probes at each TTL level and displays the round-trip time for each.
The trace command sends out one probe at a time. Each outgoing packet may result in one or two error messages. A time exceeded error message indicates that an intermediate router has seen and discarded the probe. A destination unreachable error message indicates that the destination node has received the probe and discarded it because it could not deliver the packet. If the timer goes off before a response comes in, trace prints an asterisk (*).
The trace command terminates when the destination responds, when the maximum TTL is exceeded, or when the user interrupts the trace with the escape sequence. By default, to invoke the escape sequence, press Ctrl-^ X, which is done by simultaneously pressing the Ctrl, Shift, and 6 keys, letting go, then pressing the X key.
Due to bugs in the IP implementation of various hosts and routers, the IP trace command may behave in odd ways.
Not all destinations will respond correctly to a probe message by sending back an ICMP port unreachable message. A long sequence of TTL levels with only asterisks, terminating only when the maximum TTL has been reached, may indicate this problem.
There is a known problem with the way some hosts handle an ICMP TTL exceeded message. Some hosts generate an ICMP message but they reuse the TTL of the incoming packet. Since this is zero, the ICMP packets do not make it back. When you trace the path to such a host, you may see a set of TTL values with asterisks (*). Eventually the TTL gets high enough that the ICMP message can get back. For example, if the host is six hops away, trace will time out on responses 6 through 11.
The following display shows sample IP trace output when a destination host name has been specified:
Router# trace ip ABA.NYC.mil
Type escape sequence to abort.
Tracing the route to ABA.NYC.mil (26.0.0.73)
1 DEBRIS.CISCO.COM (131.108.1.6) 1000 msec 8 msec 4 msec
2 BARRNET-GW.CISCO.COM (131.108.16.2) 8 msec 8 msec 8 msec
3 EXTERNAL-A-GATEWAY.STANFORD.EDU (192.42.110.225) 8 msec 4 msec 4 msec
4 BB2.SU.BARRNET.NET (131.119.254.6) 8 msec 8 msec 8 msec
5 SU.ARC.BARRNET.NET (131.119.3.8) 12 msec 12 msec 8 msec
6 MOFFETT-FLD-MB.in.MIL (192.52.195.1) 216 msec 120 msec 132 msec
7 ABA.NYC.mil (26.0.0.73) 412 msec 628 msec 664 msec
Table 16-16 describes the fields shown in the display.
| Field | Description |
|---|---|
| 1 | Indicates the sequence number of the router in the path to the host. |
| DEBRIS.CISCO.COM | Host name of this router. |
| 131.108.1.61 | Internet address of this router. |
| 1000 msec 8 msec 4 msec | Round-trip time for each of the three probes that are sent. |
Table 16-17 describes the characters that can appear in trace output.
| Char | Description |
|---|---|
| nn msec | For each node, the round-trip time in milliseconds for the specified number of probes. |
| * | The probe timed out. |
| ? | Unknown packet type. |
| Q | Source quench. |
| P | Protocol unreachable. |
| N | Network unreachable. |
| U | Port unreachable. |
| H | Host unreachable. |
trace (privileged)
Use the trace privileged EXEC command to discover the routes the router's packets will actually take when traveling to their destination.
trace [destination]| destination | (Optional) Destination address or host name on the command line. The default parameters for the appropriate protocol are assumed and the tracing action begins. |
Privileged EXEC
The trace command works by taking advantage of the error messages generated by routers when a datagram exceeds its time-to-live (TTL) value.
The trace command starts by sending probe datagrams with a TTL value of one. This causes the first router to discard the probe datagram and send back an error message. The trace command sends several probes at each TTL level and displays the round-trip time for each.
The trace command sends out one probe at a time. Each outgoing packet may result in one or two error messages. A time exceeded error message indicates that an intermediate router has seen and discarded the probe. A destination unreachable error message indicates that the destination node has received the probe and discarded it because it could not deliver the packet. If the timer goes off before a response comes in, trace prints an asterisk (*).
The trace command terminates when the destination responds, when the maximum TTL is exceeded, or when the user interrupts the trace with the escape sequence. By default, to invoke the escape sequence, press Ctrl-^ X, which is done by simultaneously pressing the Ctrl, Shift, and 6 keys, letting go, then pressing the X key.
To use nondefault parameters and invoke an extended trace test, enter the command without a destination argument. You will be stepped through a dialog to select the desired parameters.
Due to bugs in the IP implementation of various hosts and routers, the IP trace command may behave in odd ways.
Not all destinations will respond correctly to a probe message by sending back an ICMP port unreachable message. A long sequence of TTL levels with only asterisks, terminating only when the maximum TTL has been reached, may indicate this problem.
There is a known problem with the way some hosts handle an ICMP TTL exceeded message. Some hosts generate an ICMP message but they reuse the TTL of the incoming packet. Since this is zero, the ICMP packets do not make it back. When you trace the path to such a host, you may see a set of TTL values with asterisks (*). Eventually the TTL gets high enough that the ICMP message can get back. For example, if the host is six hops away, trace will time out on responses 6 through 11.
The following display shows sample IP trace output when a destination host name has been specified:
Router# trace ABA.NYC.mil
Type escape sequence to abort.
Tracing the route to ABA.NYC.mil (26.0.0.73)
1 DEBRIS.CISCO.COM (131.108.1.6) 1000 msec 8 msec 4 msec
2 BARRNET-GW.CISCO.COM (131.108.16.2) 8 msec 8 msec 8 msec
3 EXTERNAL-A-GATEWAY.STANFORD.EDU (192.42.110.225) 8 msec 4 msec 4 msec
4 BB2.SU.BARRNET.NET (131.119.254.6) 8 msec 8 msec 8 msec
5 SU.ARC.BARRNET.NET (131.119.3.8) 12 msec 12 msec 8 msec
6 MOFFETT-FLD-MB.in.MIL (192.52.195.1) 216 msec 120 msec 132 msec
7 ABA.NYC.mil (26.0.0.73) 412 msec 628 msec 664 msec
Table 16-18 describes the fields shown in the display.
| Field | Description |
|---|---|
| 1 | Indicates the sequence number of the router in the path to the host. |
| DEBRIS.CISCO.COM | Host name of this router. |
| 131.108.1.61 | Internet address of this router. |
| 1000 msec 8 msec 4 msec | Round-trip time for each of the three probes that are sent. |
The following display shows a sample trace session involving the extended dialog of the trace command:
Router# trace
Protocol [ip]:
Target IP address: mit.edu
Source address:
Numeric display [n]:
Timeout in seconds [3]:
Probe count [3]:
Minimum Time to Live [1]:
Maximum Time to Live [30]:
Port Number [33434]:
Loose, Strict, Record, Timestamp, Verbose[none]:
Type escape sequence to abort.
Tracing the route to MIT.EDU (18.72.2.1)
1 ICM-DC-2-V1.ICP.NET (192.108.209.17) 72 msec 72 msec 88 msec
2 ICM-FIX-E-H0-T3.ICP.NET (192.157.65.122) 80 msec 128 msec 80 msec
3 192.203.229.246 540 msec 88 msec 84 msec
4 T3-2.WASHINGTON-DC-CNSS58.T3.ANS.NET (140.222.58.3) 84 msec 116 msec 88 msec
5 T3-3.WASHINGTON-DC-CNSS56.T3.ANS.NET (140.222.56.4) 80 msec 132 msec 88 msec
6 T3-0.NEW-YORK-CNSS32.T3.ANS.NET (140.222.32.1) 92 msec 132 msec 88 msec
7 T3-0.HARTFORD-CNSS48.T3.ANS.NET (140.222.48.1) 88 msec 88 msec 88 msec
8 T3-0.HARTFORD-CNSS49.T3.ANS.NET (140.222.49.1) 96 msec 104 msec 96 msec
9 T3-0.ENSS134.T3.ANS.NET (140.222.134.1) 92 msec 128 msec 92 msec
10 W91-CISCO-EXTERNAL-FDDI.MIT.EDU (192.233.33.1) 92 msec 92 msec 112 msec
11 E40-RTR-FDDI.MIT.EDU (18.168.0.2) 92 msec 120 msec 96 msec
12 MIT.EDU (18.72.2.1) 96 msec 92 msec 96 msec
Table 16-19 describes the fields that are unique to the extended trace sequence, as shown in the display.
| Field | Description |
|---|---|
| Target IP address | You must enter a host name or an IP address. There is no default. |
| Source address | One of the interface addresses of the router to use as a source address for the probes. The router will normally pick what it feels is the best source address to use. |
| Numeric display | The default is to have both a symbolic and numeric display; however, you can suppress the symbolic display. |
| Timeout in seconds | The number of seconds to wait for a response to a probe packet. The default is 3 seconds. |
| Probe count | The number of probes to be sent at each TTL level. The default count is 3. |
| Minimum Time to Live [1] | The TTL value for the first probes. The default is 1, but it can be set to a higher value to suppress the display of known hops. |
| Maximum Time to Live [30] | The largest TTL value that can be used. The default is 30. The trace command terminates when the destination is reached or when this value is reached. |
| Port Number | The destination port used by the UDP probe messages. The default is 33434. |
| Loose, Strict, Record, Timestamp, Verbose | IP header options. You may specify any combination. The trace command issues prompts for the required fields. Note that trace will place the requested options in each probe; however, there is no guarantee that all routers (or end nodes) will process the options. |
| Loose Source Routing | Allows you to specify a list of nodes that must be traversed when going to the destination. |
| Strict Source Routing | Allows you to specify a list of nodes that must be the only nodes traversed when going to the destination. |
| Record | Allows you to specify the number of hops to leave room for. |
| Timestamp | Allows you to specify the number of time stamps to leave room for. |
| Verbose | If you select any option, the verbose mode is automatically selected and trace prints the contents of the option field in any incoming packets. You can prevent verbose mode by selecting it again, toggling its current setting. |
Table 16-20 describes the characters that can appear in trace output.
| Char | Description |
|---|---|
| nn msec | For each node, the round-trip time in milliseconds for the specified number of probes. |
| * | The probe timed out. |
| ? | Unknown packet type. |
| Q | Source quench. |
| P | Protocol unreachable. |
| N | Network unreachable. |
| U | Port unreachable. |
| H | Host unreachable. |
trace (user)
Use the transmit-interface interface configuration command to assign a transmit interface to a receive-only interface. This is used commonly with microwave Ethernet links. The no form of the command reverts both interfaces to normal duplex Ethernet interfaces.
transmit-interface interface-name| interface-name | Transmit interface to be linked with the (current) receive-only interface |
Disabled
Interface configuration
The following example specifies interface Ethernet 0 as a simplex Ethernet interface:
interface ethernet 1
ip address 128.9.1.2
transmit-interface ethernet 0
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