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SLIP and PPP define methods of sending Internet Protocol (IP) packets over standard EIA/TIA-232 asynchronous serial lines with minimum line speeds of 1200 baud.
Using SLIP or PPP encapsulation over asynchronous lines is an inexpensive way of connecting PCs to a network. SLIP and PPP over asynchronous dial-up modems allow a home computer to be connected to a network without the cost of a leased line. Dial-up SLIP and PPP links can also be used for remote sites that need only occasional telecommuting or backup connectivity. Both public-domain and vendor-supported SLIP and PPP implementations are available for a variety of computer applications.
Use the commands in this chapter to configure SLIP and PPP on your communication server. For configuration information and examples, refer to the Access and Communication Servers Configuration Guide.
See the Cisco Access Connection Guide for information about SLIP and PPP user-level EXEC connection commands.
To set the address used on the remote (PC) side, use the async default ip address interface configuration command. To remove the default address from your configuration, use the no form of this command.
async default ip address address| address | Address of the client interface |
No default address is specified.
Interface configuration
The following example specifies address 182.32.7.51 on async interface 6:
line 20
speed 19200
interface async 6
async default ip address 182.32.7.51
async dynamic address
To specify dynamic asynchronous addressing, use the async dynamic address interface configuration command. To disable dynamic addressing, use the no form of this command.
async dynamic addressThis command has no arguments or keywords.
Dynamic addressing is disabled.
Interface configuration
You can control whether addressing is dynamic (the user specifies the address at the EXEC level when making the connection), or whether default addressing is used (the address is forced by the system). If you specify dynamic addressing, the communication server must be in interactive mode and the user will enter the address at the EXEC level.
It is common to configure an asynchronous interface to have a default address and to allow dynamic addressing. With this configuration, the choice between the default address or a dynamic addressing is made by the user when they enter the slip or ppp EXEC command. If the user enters an address, it is used, and if the user enters the default keyword, the default address is used.
The following example shows dynamic addressing assigned to async interface 6.
Interface ethernet 0
ip address 1.0.0.1 255.0.0.0
interface async 6
async dynamic address
async default ip address
To allow the use of routing protocols on an interface, use the async dynamic routing interface configuration command. To disable the use of routing protocols, use the no form of this command.
async dynamic routingThis command has no arguments or keywords.
Dynamic routing is disabled.
Interface configuration
The use of routing protocols is further controlled by the use of the /routing keyword in the slip and ppp EXEC command. Refer to the Cisco Access Connection Guide for more information about making SLIP and PPP connections.
The following example shows how to enable asynchronous routing on async interface 6. The ip tcp header-compression passive command enables Van Jacobson TCP header compression and prevents transmission of compressed packets until a compressed packet arrives from the asynchronous link.
interface async 6
async dynamic routing
async dynamic address
async default ip address 1.1.1.2
ip tcp header-compression passive
ip unnumbered ethernet 0
async dynamic address
ip tcp header-compression
To place a line into dedicated asynchronous mode using SLIP or PPP encapsulation, use the async mode dedicated interface configuration command. To return the line to interactive mode, use the no form of this command.
async mode dedicatedThis command has no arguments or keywords.
Asynchronous mode is disabled.
Interface configuration
With dedicated asynchronous network mode, the interface will use either SLIP or PPP encapsulation, depending on which encapsulation method is configured for the interface. An EXEC prompt does not appear, and the communication server is not available for normal interactive use.
If you configure a line for dedicated mode, you will not be able to use the async dynamic address command, because there is no user prompt.
The following example assigns an IP address to an asynchronous line and places the line into network mode. Setting the stop bits to 1 enhances performance.
interface async 4
async default ip address 182.32.7.51
async mode dedicated
encapsulation slip
line 20
location Joe's computer
stopbits 1
speed 19200
async mode interactive
To return a line that has been placed into dedicated asynchronous network mode to interactive mode, thereby enabling the slip and ppp EXEC commands, use the async mode interactive interface configuration command. To prevent users from implementing SLIP and PPP at the EXEC level, use the no form of this command.
async mode interactiveThis command has no arguments or keywords.
Asynchronous mode is disabled.
Interface configuration
Interactive mode enables the slip and ppp EXEC commands. In dedicated mode, there is no user EXEC level. The user does not enter any commands, and a connection is automatically established when the user logs on, according to the configuration.
The following example places async interface 6 into interactive asynchronous mode:
interface async 6
async default ip address 182.32.7.51
async mode interactive
ip unnumbered ethernet 0
async mode dedicated
To support the extended BOOTP request specified in RFC 1084, and to specify information that will be sent in response to BOOTP requests, use the async-bootp global configuration command. To clear the list, use the no form of this command.
async-bootp tag [:hostname] data| tag | Item being requested; expressed as filename, integer, or IP dotted decimal address. See Table 15-1 for possible values. |
| :hostname | (Optional) This entry applies only to the specified host. The argument can be either an IP address or a logical host name. |
| data | List of IP addresses entered in dotted decimal notation or as logical host names, a number, or a quoted string. |
If no extended BOOTP commands are entered, the software generates a gateway and subnet mask appropriate for the local network.
Global configuration
Each of the tag keyword-argument pairs is a field that can be filled in and sent in response to BOOTP requests from clients.
BOOTP supports the extended BOOTP requests specified in RFC 1084 and works for both SLIP and PPP encapsulation.
Use the show async bootp EXEC command to list the configured parameters. BOOTP works for both SLIP and PPP.
The following example specifies different boot files: one for a PC and one for a Macintosh. With this configuration, a BOOTP request from the host on 128.128.1.1 results in a reply listing the boot filename as pcboot. A BOOTP request from the host named mac results in a reply listing the boot filename as macboot.
async-bootp bootfile :128.128.1.1 "pcboot"
async-bootp bootfile :mac "macboot"
The following example specifies a subnet mask of 255.255.0.0:
async-bootp subnet-mask 255.255.0.0
The following example specifies a negative time offset of the local subnetwork of -3600 seconds:
async-bootp time-offset -3600
The following example specifies the IP address of a time server:
async-bootp time-server 128.128.1.1
show async-bootp
To return a line to its idle state, enter the clear line privileged EXEC command at the system prompt.
clear line line-number| line-number | Asynchronous line port number assigned with the interface async command |
Privileged EXEC
Normally, this command returns the line to its conventional function as a terminal line, with the interface left in a "down" state.
The following example shows how to use the clear line command to return serial interface 5 to its idle state:
clear line 5
To debug asynchronous interfaces, use the debug async privileged EXEC command. The undebug command turns off the debugging function.
debug async {framing | state | packets}| framing | Displays errors that have occurred in the framing of asynchronous packets. Includes CRC errors and illegal sequence errors, as well as packet length errors. |
| state | Displays changes in the asynchronous interface state, such as protocol mode being turned on and off. |
| packets | Displays log message for each input and output packet on asynchronous interfaces. This keyword creates a lot of output; use with care. |
Disabled
Privileged EXEC
To debug PPP, use the debug ppp privileged EXEC command. To turn off the debugging function, use the undebug command.
debug ppp {negotiation | error | packet | chap}| negotiation | Debugs the PPP protocol negotiation process. |
| error | Displays PPP protocol errors and error statistics. |
| packet | Displays PPP protocol messages sent and received. |
| chap | Displays errors encountered during remote or local system authentication. |
Disabled
Privileged EXEC
The command debug ppp packet creates a lot of output. Use with care.
To configure SLIP or PPP encapsulation as the default on an asynchronous interface, use the encapsulation interface configuration command. To disable encapsulation, use the no form of this command.
encapsulation {slip | ppp}| slip | Specifies SLIP encapsulation for an interface configured for dedicated asynchronous mode or DDR. |
| ppp | Specifies PPP encapsulation for an interface configured for dedicated asynchronous mode or DDR. |
SLIP encapsulation is enabled by default.
Interface configuration
On lines configured for interactive use, encapsulation is selected by the user when they establish a connection with the slip or ppp EXEC command.
IP Control Protocol (IPCP) is the part of PPP that brings up and configures IP links. After devices at both ends of a connection communicate and bring up PPP, they bring up the control protocol for each network protocol they intend to run over the PPP link such as IP or IPX. If you have problems passing IP packets and the show interface command shows that line is up, use the debug ppp negotiations debugging command to see if and where the negotiations are failing. You might have different versions of software running, or different versions of PPP, in which case you might need to upgrade your software or turn off PPP option negotiations. All IPCP options as listed in RFC 1332 are supported on asynchronous lines. Only Option 2, TCP/IP header compression, is supported on synchronous interfaces.
PPP echo requests are used as keepalives to detect line failure. The no keepalive command can be used to disable echo requests. For more information about the no keepalive command, refer to the chapter "IP Routing Protocols Commands" later in this publication and the chapter "Configuring IP Routing Protocols" in the Access and Communication Servers Configuration Guide publication.
In order to use SLIP or PPP, the communication server must be configured with an IP routing protocol or with the ip host-routing command. This configuration is done automatically if you are using old-style slip address commands. However, you must configure it manually if you configure SLIP or PPP via the interface async command.
In the following example, async interface 1 is configured for PPP encapsulation.
tarmac# config
Configuring from terminal, memory, or network [terminal]?
Enter configuration commands, one per line. End with CNTL/Z.
tarmac(config)# interface async 1
tarmac(config-if)# encapsulation ppp
A dagger (+) indicates that the command is documented in another chapter.
keepalives +
debug ppp
To limit the size of the IP output queue, use the hold-queue interface configuration command. To return the output queue to the default size, use the no form of this command.
hold-queue packets| packets | Maximum number of packets. The range of values is 0 through 65535. |
10 packets (default for asynchronous interfaces only)
Interface configuration
The default of 10 packets allows the communication server to queue a number of back-to-back routing updates. This is the default for asynchronous interfaces only; other media types have different defaults.
The hold queue stores packets received from the network that are waiting to be sent to the client. It is recommended that the queue size not exceed ten packets on asynchronous interfaces. For most other interfaces, queue length should not exceed 100.
The following example changes the packet queue length of a line to five packets:
interface async 2
async default ip address 182.32.7.5
hold-queue 5
To specify the interface you want to configure, use the interface global configuration command. To clear the interface configuration, use the no form of this command.
interface type number| type | Interface type. |
| number | Interface number. See Table 15-2 for a list of interface numbers by communication server model. |
| Communication Server Model | Interface Number |
|---|---|
| 508-CS | 1 to 8 |
| 516-CS | 1 to 16 |
| ASM-CS (fully configured) | 1 to 113 |
| 2509 or 2510 | 1 to 8 |
| 2511 or 2512 | 1 to 16 |
No interface is specified by default; you must specify an interface to configure it.
Global configuration
The following example specifies async interface 1:
interface async 1
To configure an access list to be used for packets transmitted to and from the asynchronous host, use the ip access-group interface configuration command. To disable control over packets transmitted to or from an asynchronous host, use the no ip access-group command.
ip access-group access-list-number {in | out}| access-list-number | Assigned IP access list number. |
| in | Defines access control on packets transmitted from the asynchronous host. |
| out | Defines access control on packets being sent to the asynchronous host. |
Disabled
Interface configuration
With this command in effect, the IP destination address of each packet is run through the access list for acceptability and dropped or passed.
The following example assumes that users are restricted to certain servers designated as SLIP or PPP servers, but that normal terminal users can access anything on the local network:
! access list for normal connections
access-list 1 permit 131.108.0.0 0.0.255.255
!
! access list for SLIP packets.
access-list 2 permit 131.108.42.55
access-list 2 permit 131.108.111.1
access-list 2 permit 131.108.55.99
!
! Specify the access list
interface async 6
async dynamic address
ip access-group 1 out
ip access-group 2 in
To set IP addresses for an interface, use the ip address interface configuration command. To remove the specified addresses, use the no ip address interface configuration command.
ip address address mask [secondary]| address | IP address. |
| mask | Network mask for the associated IP network. |
| secondary | (Optional) Specifies additional IP addresses. |
No IP addresses are specified.
Interface configuration
The subnet mask must be the same for all interfaces connected to subnets of the same network. Hosts can determine subnet masks using the Internet Control Message Protocol (ICMP) Mask Request message. Communication servers 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 interface configuration command. If the router detects another host using one of its IP addresses, it will print an error message on the console.
In the example that follows, 131.108.1.27 is the primary address and 192.31.7.17 and 192.31.8.17 are secondary addresses for async interface 1:
interface async 1
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
To specify the size of the largest Internet packet, use the ip mtu interface configuration command. To return to the default MTU size of 1500 bytes, use the no form of this command.
ip mtu bytes| bytes | Maximum number of bytes. The range of values is 64 to 1000000. |
1500 bytes
Interface configuration
The following example sets the packet MTU size to 200 bytes:
interface async 5
async default ip address 182.32.7.5
ip mtu 200
To configure Van Jacobson TCP header compression on the asynchronous link, use the ip tcp header-compression line configuration command. To disable header compression, use the no form of this command.
ip tcp header-compression [on | off | passive]| on | (Optional) Turns header compression on. |
| off | (Optional) Turns header compression off. |
| passive | (Optional) On SLIP lines, prevents transmission of compressed packets until a compressed packet arrives from the asynchronous link, unless a user specifies SLIP on the command line. For PPP, this option functions the same as the on option. |
Header compression is on.
Interface configuration
Header compression data areas are initialized to handle up to 16 simultaneous TCP connections. Currently, you cannot change this number. You can only turn header compression on or off or use the passive keyword.
On lines configured for PPP encapsulation, the keywords passive and on cause the same behavior because, before attempting header compression, PPP automatically negotiates whether it is available at each end of the connection.
There are two ways to implement header compression when the line is configured for ip tcp header-compression passive:
If a line is configured for passive header compression and you use the slip or ppp EXEC command to enter asynchronous mode, you will see that the interface is set to match compression status used by the host at the other end of the asynchronous line.
Server> slip 1.0.0.1
Password:
Entering SLIP mode.
Interface IP address is 1.0.0.1, MTU is 1500 bytes
Header compression will match your system.
The message "Header compression will match your system" indicates that the interface is set to match the compression status used by the host at the other end of the asynchronous line. If the line was configured to have header compression on, this line would read "Header compression is On." Refer to the Cisco Access Connection Guide for more information about making SLIP and PPP connections.
The following example illustrates how to enable Van Jacobson TCP header compression. The passive keyword prevents transmission of compressed packets until a compressed packet arrives from the IP link. Notice that asynchronous routing and dynamic addressing are also enabled.
interface async 6
async dynamic routing
async dynamic address
ip tcp header-compression passive
Refer to the Cisco Access Connection Guide for documentation on these commands:
ppp
slip
slip default
slip /compressed
To conserve network resources, use the ip unnumbered line configuration command. To disable unnumbered interfaces, use the no form of this command.
ip unnumbered type number| type | Interface type. |
| number | Interface number. |
Disabled
Interface configuration
You must use either the ip address or ip unnumbered command to provide the local address for an interface.
Unnumbered interfaces do not have an address. Network resources are conserved because fewer network numbers are used and routing tables are smaller.
Whenever the unnumbered interface generates a packet (for example, 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 to determine which routing processes are sending updates over the unnumbered interface. Restrictions include the following:
The following example shows how to configure async interface 6 as unnumbered:
interface async 6
ip unnumbered ethernet 0
ip address
To display the parameters that have been configured for extended BOOTP requests, use the show async bootp privileged EXEC command.
show async bootpThis command has no arguments or keywords.
Privileged EXEC
The following is sample output from the show async bootp command.
sloth# show async bootp
The following extended data will be sent in BOOTP responses:
bootfile (for address 128.128.1.1) "pcboot"
bootfile (for address 131.108.1.111) "dirtboot"
subnet-mask 255.255.0.0
time-offset -3600
time-server 128.128.1.1
Table 15-3 describes significant fields shown in the display.
| Field | Description |
|---|---|
| bootfile... "pcboot" | Indicates that the boot file for address 128.128.1.1 is named pcboot. |
| subnet-mask 255.255.0.0 | Specifies the subnet mask. |
| time-offset -3600 | Indicates that the local time is one hour (3600 seconds) earlier than Coordinated Universal Time (UTC). |
| time-server 128.128.1.1 | Indicates the address of the time server for the network. |
To display the status of activity on all lines configured for asynchronous support, use the show async status privileged EXEC command.
show async statusThis command has no arguments or keywords.
Privileged EXEC
The display resulting from this command shows all asynchronous sessions, whether they are using SLIP or PPP encapsulation.
The following is sample output from the show async status command:
cs# show async status
Async protocol statistics:
Rcvd: 5448 packets, 7682760 bytes
1 format errors, 0 checksum errors, 0 overrun, 0 no buffer
Sent: 5455 packets, 7682676 bytes, 0 dropped
Tty Local Remote Qd InPack OutPac Inerr Drops MTU Qsz
1 192.31.7.84 Dynamic 0 0 0 0 0 1500 10
* 3 192.31.7.98 None 0 5448 5455 1 0 1500 10
Table 15-4 describes significant fields shown in the display.
| Field | Description |
|---|---|
| Rcvd: | Statistics on packets received. |
| 5448 packets | Packets received. |
| 7682760 bytes | Total number of bytes. |
| 1 format errors | Spurious characters received when a packet start delimiter is expected. |
| 0 checksum errors | Count of checksum errors. |
| 0 overrun | Number of giants received. |
| 0 no buffer | Number of packets received when no buffer was available. |
| Sent | Statistics on packets sent. |
| 5455 packets | Packets sent. |
| 7682676 bytes | Total number of bytes. |
| 0 dropped | Number of packets dropped. |
| Tty | Line number. |
| * | Line currently in use. |
| Local | Local IP address on the link. |
| Remote | Remote IP address on the link; "Dynamic" indicates that a remote address is allowed but has not been specified; "None" indicates that no remote address is assigned or being used. |
| Qd | Number of packets on hold queue (Qsz is the maximum). |
| InPack | Number of packets received. |
| OutPac | Number of packets sent. |
| Inerr | Number of total input errors; sum of format errors, checksum errors, overruns and no buffers. |
| Drops | Number of packets received that would not fit on the hold queue. |
| MTU | Current maximum transmission unit size. |
| Qsz | Current output hold queue size. |
Use the show line privileged EXEC command to display connection status for a line running in asynchronous mode.
show line [line-number]| line-number | (Optional) Particular line about which information will be displayed. If you do not specify a line number, information about all lines is displayed. |
Command Mode
EXEC
The following is sample output from the show line command:
mosey> show line
Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns
* 0 CTY - - - - - 0 0 0/0
A 1 TTY 9600/9600 - - - - 1 0 0 0/0
2 TTY 9600/9600 - - - - - 0 0 0/0
3 TTY 9600/9600 - - - - - 0 0 0/0
4 TTY 9600/9600 - - - - - 0 0 0/0
5 TTY 9600/9600 - - - - - 0 0 0/0
6 TTY 9600/9600 - - - - - 0 0 0/0
7 TTY 9600/9600 - - - - - 0 0 0/0
8 TTY 9600/9600 - - - - - 0 0 0/0
9 TTY 9600/9600 - - - - - 0 0 0/0
10 TTY 9600/9600 - - - - - 0 0 0/0
11 TTY 9600/9600 - - - - - 0 0 0/0
12 TTY 9600/9600 - - - - - 0 0 0/0
13 TTY 9600/9600 - - - - - 0 0 0/0
14 TTY 9600/9600 - - - - - 0 0 0/0
15 TTY 9600/9600 - - - - - 0 0 0/0
16 TTY 9600/9600 - - - - - 0 0 0/0
* 17 VTY 9600/9600 - - - - - 18 0 0/0
Table 15-5 describes significant fields shown in the display.
| Tasks | Descriptions |
|---|---|
| (first character in line) | The field preceding the number in the Tty field can be blank or contain one of the following characters:
* The line is currently active, running a terminal-oriented protocol A The line is currently active in asynchronous mode I The line is free and can be used for asynchronous modes because it is |
| Tty | Indicates the absolute line number of the specified line. |
| Typ | Type of line. Possible values follow:
CTY--Console AUX--Auxiliary port TTY--Asynchronous terminal port VTY--Virtual terminal LPT--Parallel printer |
| Tx/Rx | Transmit rate of the line (baud)/receive rate of the line (baud). |
| A | Indicates whether or not autobaud has been configured for the line. A value of F indicates that autobaud has been configured; a hyphen (-) indicates that it has not been configured for the line. |
| Modem | Types of modem signal that has been configured for the line. Possible values include:
callin callout cts-req DTR-Act inout RIisCD |
| Roty | Rotary Group configured for this line. |
| AccO | Output access list number configured for the specified line. |
| AccI | Input access list number configured for the specified line. |
| Uses | Number of connections established to or from this line since the system was restarted. |
| Noise | Number of times noise has been detected on the line since the system restarted. |
| Overruns | Hardware (UART) overruns/software buffer overflows, both defined as the number of overruns or overflows that have occurred on the specified line since the system was restarted. Hardware overruns are buffer overruns; the UART chip has received bits from the software faster than it can process them. A software overflow occurs when the software has received bits from the hardware faster than it can process them. |
The following is sample output from the show line command when a line is specified:
cs> show line 1
Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns
1 TTY 9600/9600 - - - - 10 0 0 0
Line 1, Location: "charnel console", Type: ""
Length: 24 lines, Width: 80 columns
Baud rate (TX/RX) is 9600/9600, no parity, 2 stopbits, 8 databits
Status: Ready, Hardware XON/XOFF
Capabilities: none
Modem state: Ready
Special Chars: Escape Hold Stop Start Disconnect Activation
^^x none - - none
Timeouts: Idle EXEC Idle Session Modem Answer Session Dispatch
0:10:00 never 0:00:15 not imp not set
Session limit is not set.
Allowed transports are telnet lat rlogin. Preferred is lat
No output characters are padded
Characters causing immediate data dispatching:
Char ASCII
Group codes: 0
async dynamic address
async dynamic routing
ip tcp header-compression
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