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This chapter describes how to configure the Serial Line Internet Protocol (SLIP) on the Cisco Systems terminal servers. You will find the following information in this chapter:
Making SLIP connections is described in the "Terminal Server User Commands" chapter.
The commands to define the configuration are entered in configuration mode, which is privileged. To enter configuration mode, type the configure command at the EXEC prompt. You can then enter the commands described in this chapter. For more information about the configuration mode, see"The Configure Command" in the "Startup and Basic Configuration" chapter.
A command summary is included at the end of the chapter.
As personal computers (PCs) become increasingly powerful, they can be connected to networks as full network nodes and not simply as dumb terminals. As a full network node, a PC can send and receive mail, transfer files using sophisticated file transfer protocols, use network file systems, form virtual terminal connections to other hosts, and so on. Several protocol suites can perform these functions, but the TCP/IP suite is preferred when a variety of computers must communicate.
The Serial Line Internet Protocol, or SLIP, is an inexpensive way of connecting PCs to a network. SLIP can be used over asynchronous dial-up modems, allowing computers in people's homes to be connected to a network without the cost of a leased line. Dial-up SLIP links can also be used for remote sites that need only occasional or backup connectivity. Both public-domain and vendor-supported SLIP implementations are available for a variety of computer applications.
SLIP defines a method of sending Internet packets over standard RS-232 asynchronous serial lines. It is a de facto standard, commonly used for point-to-point serial connections running TCP/IP. SLIP is commonly used on dedicated serial links with line speeds between 1,200 and 19,200 baud.
The version of SLIP described in this manual was originally implemented by researchers at the University of California at Berkeley in their 4.2 BSD version of the UNIX operating system. Although variants have been proposed, the Berkeley version has emerged as a de facto standard. Refer to RFC1055 for more information about SLIP.
The Cisco Systems terminal servers combine the ability to run SLIP with hardware and software optimized to handle terminal line traffic. The terminal server concentrates a large number of SLIP PC hosts onto a network interface, allowing the PCs to communicate to any host on the network. The terminal server can support any combination of SLIP lines and lines dedicated to normal asynchronous devices, such as terminals and modems.
Figure 1-1 illustrates a typical SLIP configuration.
In addition to implementing the de facto standard, Cisco's implementation of SLIP offers both dedicated and dynamic address assignment, configurable hold queue and IP packet sizes, extended BootP requests, and permit/deny conditions for controlling access to the line.
The Cisco Systems terminal servers recognize a variety of Internet broadcast addresses. When a terminal server receives an Internet packet with one of these addresses from a SLIP client, it rebroadcasts the packet onto the network without changing the Internet header. The terminal server does not alter the packet's broadcast address to match the form of broadcast address it prefers.
The terminal server receives a copy of SLIP client broadcasts, and responds to BootP requests with the Internet address of the line that received them. This facility allows the SLIP client software to automatically determine its own Internet address.
Following are the basic steps for configuring SLIP on the Cisco terminal server.
Step 1: Enable SLIP on a terminal server line using the slip dedicated, slip address, or slip address dynamic line subcommands. See the section "SLIP Connection Types" for more information about selecting these commands.
Step 2: Make the appropriate settings for the line. Line settings include baud rate, flow control, stop bits, and modem control. These settings are described in the sections "Configuring Asynchronous Lines" and "Configuring the Modem Control Lines" in the "System Configuration" chapter.
Step 3: Specify packet size limits and access lists for control of traffic to or from a SLIP-enabled line, if needed.
Step 4: Specify extended BootP requests, if needed.
Additionally, SLIP supports simultaneous routing of packets to networks connected via SLIP lines on the Cisco TRouter, the combination terminal server/router product.
The following sections describe how to configure the terminal server for SLIP operation. Descriptions of the EXEC commands used to monitor and maintain a SLIP link follow these sections.
Users use one of two EXEC commands to make a SLIP connection, depending on how the line is configured:
slipIt is also possible to configure a dedicated SLIP line, in which case no EXEC command is required to make the connection.
The following paragraphs describe the ways in which the SLIP lines can be configured.
A line can be permanently configured for SLIP using the slip dedicated and slip address line configuration commands. In this case, an EXEC is not started on the line, so the user need not enter a command to initiate SLIP. They need only to make the physical connection to the line and then are immediately placed in SLIP mode. No prompt or message is issued.
A line can be assigned a permanent SLIP address with the slip address line configuration command. In this case, the user issues the slip EXEC command to put the line into SLIP mode. If the server has been configured to authenticate SLIP connections, the user is prompted for a password before the line is placed in SLIP mode.
A line can be configured for dynamic assignment of SLIP addresses with the slip address dynamic command. In this case, the user enters the slip EXEC command and is prompted for the IP address or logical host name to use. This address is validated via TACACS (when enabled) and the line is put into SLIP mode using the address requested.
This feature is useful in a situation where the user has a need to know the IP address of a line. A personal computer running an application that automatically dials in using SLIP and polls for electronic mail messages would be an example of this. The application can be set up to dial in periodically and enter the required IP address and password.
A line can also be given a default address to use. In this case, the user issues the slip default EXEC command, the transaction is validated by the TACACS server (when enabled), and the line is put into SLIP mode using the address configured with the IP address argument of the slip address dynamic configuration command.
This feature is useful when it is not reasonable for all users to know the IP address to use to gain access to a system, for example, a server that is available to many students on a campus would be an example of this. Instead of requiring each to know an IP address, they need only enter the EXEC slip default command and let the server select the line to use.
Once a line is configured for SLIP, the EXEC responds to the slip or slip default EXEC commands by turning on SLIP on the line, displaying the Internet address and the size of the largest Internet packet the SLIP support can handle. The line exits SLIP mode when the modem is hung up or an EXEC clear line command is issued.
The no slip line subcommand disables SLIP mode. It has this syntax:
no slipUse this command to disable SLIP on a line that has previously had SLIP enabled.
The slip address line subcommand specifies the Internet address assigned to the SLIP client at the other end of the serial line connection. The command has this syntax:
slip address internet-addressThe argument internet-address must be on the same network or subnet as one of the terminal server's network interfaces.
To put this line into SLIP mode, use the EXEC command slip; see "SLIP Connection Types" in this chapter.
This example sets IP address 182.32.7.51 on line 2.
line 2
slip address 182.32.7.51
The slip dedicated line subcommand puts the line in SLIP mode permanently. It has this simple syntax:
slip dedicatedThe terminal server will not create an EXEC on this line, so it is not available for normal interactive use. No slip EXEC command is necessary to enable SLIP mode.
The slip dedicated statement permanently places the line into SLIP mode.
line 2
slip address 182.32.7.5
slip dedicated
The slip interactive line subcommand allows a line to be used in either SLIP mode or
interactive mode.
The slip interactive subcommand is generally used to void a slip dedicated line subcommand. It is the equivalent to the command no slip dedicated (although there is no such command). To put this line into SLIP mode, use the EXEC command slip; see "SLIP Connection Types" in this chapter.
Hanging up the modem or clearing the line puts the line back into interactive mode.
Dynamic address assignment requires that the user enter an IP address to enter SLIP mode. The full syntax of the line subcommand to set this up follows:
slip address dynamicThis subcommand allows flexibility when the lines are in a rotary configuration, when you cannot predict to which line a user will connect. To put this line into SLIP mode, use the EXEC command slip.
This feature is supported when a TACACS server is used. (The host name sent in a TACACS request will be in all uppercase letters.)
In the following example, line 3 is configured for dynamic address assignment:
line 3
slip address dynamic
In this example, the range of lines 10 through 15 are configured for dynamic address
assignment.
line 10 15
slip address dynamic
Lines configured for dynamic address assignment can also be given a default address to use so the user enters the EXEC command slip default to make connection. The syntax for this is as follows:
slip address dynamic IP-addressThe argument IP-address is the IP address to use.
This example illustrates how to configure a default SLIP address.
line 1
slip address dynamic 108.91.3.4
The slip hold-queue line subcommand specifies the limit of the SLIP output queue, which stores packets received from the network waiting to be sent to the SLIP client.
slip hold-queue packetsThe argument packets is the maximum number of packets. The default is three packets; it is recommended that the queue size does not exceed 10.
This command changes the packet queue length to five packets.
line 5
slip address 182.32.7.5
slip hold-queue 5
The slip mtu line subcommand specifies the size of the largest Internet packet that the SLIP support can handle.
slip mtu bytesThe argument bytes is the maximum number of bytes. The default is 1500 MTU.
You might want to change to a smaller MTU size if the SLIP application at the other end does not support packets of that size, or you want to assure a lower delay by using shorter packets. This can be desirable when the host Telnet echoing takes longer than 0.2 seconds. For instance, at 9600 baud, a 1500 byte packet takes about 1.5 seconds to transmit, so this delay would indicate that you want an MTU size of about 200.
On the other hand, the MTU size can be negotiated by TCP, regardless of what the terminal settings are, and this is the better way to do it. The terminal server performs IP fragmentation of packets larger than the specified MTU. Therefore, do not use this command unless the SLIP implementation supports re-assembly of IP fragments. Since each fragment occupies a spot in the output queue, it may also be necessary to increase the size of the SLIP hold queue.
These commands set the packet MTU size to 200 bytes.
line 5
slip address 182.32.7.5
slip mtu 200
Access lists allow the system administrator to control the hosts that may be accessed by a PC running SLIP through a terminal server. In versions of the software prior to 8.3, turning on SLIP defeated any access lists that were defined for that line. As SLIP is different than a
connection, separate access lists may be defined for SLIP and for normal connections. The software allows separate access lists to be defined for use when the line is running SLIP.
To configure an access list to be used on packets from the SLIP host, use this line subcommand:
slip access-class number inWhen this command is entered, the IP destination address of each packet is run through the access list for acceptability, and dropped or passed. The argument number is the IP access list number (see the section "Configuring IP Access Lists" in the "TCP/IP Configuration ansd Management" chapter, for information about IP access lists).
To specify an access list to be used on packets being sent to the SLIP host, use this line subcommand:
slip access-class number outWhen this command is entered, the IP source address is compared against the access list, and only those packets allowed by the access list are transmitted on the asynchronous line. The argument number is the IP access list number (see the section "Configuring IP Access Lists" in the "TCP/IP Configuration ansd Management" chapter, for information about IP access lists).
On the TRouter, it is also possible to use extended IP access lists on SLIP lines. In this case, the header of the packet is compared against the access list as appropriate.
This example assumes that SLIP users are restricted to certain servers designated as SLIP 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
!
!Define all the lines with slip addresses and
!appropriate access lists
line 1 60
slip address dynamic
access-class 1 out
slip access-class 2 in
!
The BootP (Boot Protocol) server for SLIP supports the extended BootP requests specified in RFC1084. These requests are specified with the async-bootp global configuration command. The full syntax for this command follows:
async-bootp tag [:hostname] data...The argument tag is the item being requested, and is one of the following expressed as file name, integer, or IP dotted decimal address:
Use the optional argument :hostname to indicate that this entry applies only to the host specified. The argument :hostname accepts both an IP address and logical host name.
The argument data can be a 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, by default the software generates a gateway and subnet mask appropriate for the local network.
Use the EXEC command show async-bootp to list the configured parameters. Use the no async-bootp command to clear the list.
This example illustrates how to specify different boot files, one for a PC, and one for a Macintosh.
async-bootp bootfile :128.128.1.1 "pcboot"
async-bootp bootfile :mac "macboot"
With this configuration, a BootP request from the host on 128.128.1.1 results in a reply listing the boot file name as pcboot. A bootP request from the host named mac results in a reply listing the boot file name as macboot.
Example 2:
This example specifies a subnet mask of 255.255.0.0.
async-bootp subnet-mask 255.255.0.0
Example 3:
This example specifies a negative time offset of the local subnetwork of -3600 seconds.
async-bootp time-offset -3600
Example 4:
This example specifies the IP address of a time server.
async-bootp time-server 128.128.1.1
The Cisco TRouter can route packets to networks connected via a SLIP line. (Earlier versions of TRouter software supported only end-nodes connected via SLIP.)
To route to a network on a SLIP line, use the ip route command in combination with the slip address line subcommand.
The following example creates a static route that is added to the router's routing table when the line enters SLIP mode. The route is deleted when SLIP is turned off on that line, such as when a modem hangs up.
line 5
slip address 130.107.4.40
ip route 10.0.0.0 130.107.4.40
Once the SLIP line is active, this route behaves identically to any other static route, and may be redistributed using any of the dynamic routing protocols. Refer to the Cisco Router Products Configuration and Reference publication for details on configuring dynamic routing protocols.
The following sample configuration assigns an Internet address to a SLIP line and puts the line in SLIP mode permanently. Setting the stop bits to 1 is a performance enhancement.
!
line 20
location Joe's computer
stopbits 1
speed 19200
slip address 182.32.7.51
slip dedicated
!
This section describes the EXEC commands for maintaining SLIP support on the terminal server.
Maintaining SLIP is a simple task on the terminal server and TRouter. The clear line EXEC command disables SLIP mode and starts an EXEC on a nondedicated SLIP line. Enter this command at the EXEC prompt:
clear line line-numberThe argument line-number specifies the line. This command is the only way to exit SLIP mode on a line without modem control.
Use the EXEC show commands described in this section to obtain displays of activity on the SLIP line.
The show ip aliases command displays Internet addresses mapped to TCP ports (aliases) and SLIP addresses, which are treated similarly to the aliases. Enter this command at the EXEC prompt:
show ip aliasesTo distinguish a SLIP address from a normal alias address, the command output uses the form SLIP TTYn for the "port" number, where n is the terminal number (in octal).
Sample output follows:
IP Address Port
192.31.7.52 SLIP TTY35
192.31.7.53 SLIP TTY36
192.31.7.54 SLIP TTY37
192.31.7.55 SLIP TTY40
The display lists the IP address and corresponding port number.
The show ip arp EXEC command displays the Address Resolution Protocol (ARP) cache, where SLIP addresses appear as permanent ARP table entries. To display the IP ARP cache, use the following EXEC command:
show ip arpAn Address Resolution Protocol 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. Following is sample output. Table 1-1 describes the fields displayed.
Protocol Address Age (min) Hardware Addr Type Interface
Internet 131.108.1.140 137 aa00.0400.6408 ARPA Ethernet0
Internet 131.108.1.111 156 0800.2007.8866 ARPA Ethernet0
Internet 131.108.1.115 33 0000.0c01.0509 ARPA Ethernet0
Internet 192.31.7.24 5 0800.0900.46fa ARPA Ethernet2
Internet 192.31.7.26 41 aa00.0400.6508 ARPA Ethernet2
Internet 192.31.7.27 - aa00.0400.0134 ARPA Ethernet2
Internet 192.31.7.28 67 0000.0c00.2c83 ARPA Ethernet2
Internet 192.31.7.17 67 2424.c01f.0711 ARPA Ethernet2
Internet 192.31.7.18 64 0000.0c00.6fbf ARPA Ethernet2
Internet 192.31.7.21 114 2424.c01f.0715 ARPA Ethernet2
Internet 131.108.1.33 15 0800.2008.c52e ARPA Ethernet0
Internet 131.108.1.55 44 0800.200a.bbfe ARPA Ethernet0
Internet 131.108.1.6 89 aa00.0400.6508 ARPA Ethernet0
Internet 131.108.7.1 - 0000.0c00.750f ARPA Ethernet3
Internet 131.108.1.1 - aa00.0400.0134 ARPA Ethernet0
| Field | Description |
|---|---|
| Protocol | Protocol for network address in Address field. |
| Address | The network address that corresponds to Hardware Addr. |
| Age (min) | Age, in minutes, of the last update of the cache entry. |
| Hardware Addr | LAN hardware address that corresponds to network address. |
| Type | Type of ARP (Address Resolution Protocol):
ARPA = Ethernet-type ARP SNAP = RFC1042 ARP Probe = HP Probe Protocol |
The show line EXEC command displays SLIP status for a line running in SLIP mode. Enter this command at the EXEC prompt:
show line line-numberThe argument line-number specifies the line.
Following is sample output:
Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise
0 CTY - - - - - 1 0
1 SLIP 9600/9600 - - - - - 0 0
2 TTY 9600/9600 - - 1 - - 0 0
3 TTY 9600/9600 - inout 1 - - 0 0
4 TTY 9600/9600 - - - - - 0 0
* 5 TTY 9600/9600 - - - - - 1 0
6 SLIP 9600/9600 - - - - - 0 0
7 TTY 9600/9600 - - - - - 0 0
10 TTY 9600/9600 - - - - - 0 0
11 TTY 9600/9600 - - - - - 0 0
12 TTY 9600/9600 - inout - - - 0 0
13 TTY 9600/9600 - - - - - 0 0
14 TTY 9600/9600 - - - - - 0 0
15 TTY 9600/9600 - - - - - 0 0
16 TTY 9600/9600 - - - - - 0 0
17 TTY 9600/9600 - - - - - 0 0
The show slip EXEC command displays the status of all lines configured for SLIP support. Enter this command at the EXEC prompt:
show slipFollowing is sample output. Table 1-2 describes the fields displayed by this command.
Slip statistics:
Rcvd: 0 packets, 0 bytes, 0 escapes
0 format errors, 0 checksum errors, 0 overrun, 0 no buffer
Sent: 0 packets, 0 bytes, 0 escapes, 0 dropped
Tty Mod Address Istate Ostate Qd InPack OutPac Inerr Dropped MTU Qsz
35 i/o 192.31.7.52 IDLE IDLE 0 0 0 0 0 1524 0
36 i/o 192.31.7.53 IDLE IDLE 0 0 0 0 0 1524 0
37 i/o 192.31.7.54 IDLE IDLE 0 0 0 0 0 1006 0
40 i/o 192.31.7.55 IDLE IDLE 0 0 0 0 0 1006 0
| Field | Description |
|---|---|
| Rcvd: | Statistics on packets received. |
| packets | Packets received. |
| bytes | Total number of bytes. |
| escapes | Count of escape characters received. |
| format errors | Packets with a bad IP header, even before the checksum is calculated. |
| checksum errors | Count of checksum errors. |
| overrun | Number of giants received. |
| no buffer | Number of packets received when no buffer was available. |
| Sent | Statistics on packets sent. |
| packets | Packets sent. |
| bytes | Total number of bytes. |
| escapes | Count of escape characters sent. |
| dropped | Number of packets dropped. |
| Tty mod | Type of modem control. |
| IState and OState | Used by Cisco engineers for troubleshooting. |
| Qd | Number of packets on hold queue (Qsz is max). |
| InPack | Number of packets input for asynchronous line. |
| OutPac | Number of packets sent to asynchronous line. |
| Inerr | Number of total input errors; sum of format errors, checksum errors, overruns and no buffers. |
| Dropped | Number of packets received that would not fit on the hold queue (time to either fix the hosts' TCPs, or increase the hold queue size). |
| * | An asterisk indicates a line currently in SLIP mode. |
The show async-bootp EXEC command displays the parameters that have been configured for SLIP extended BootP requests. Enter this command at the EXEC prompt:
show async-bootpFollowing is sample output:
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
The privileged-mode EXEC debug commands described in this section are used to troubleshoot lines in SLIP mode. Generally, you enter these commands during troubleshooting sessions with Cisco staff.
For each debug command, there is a corresponding undebug command that turns the display off. Entering the debug commands will affect performance of the SLIP feature.
The debug slip EXEC command enables logging of all SLIP activity. The high volume of SLIP debugging output, which amounts to several lines per packet, noticeably affects overall system performance.
The debug slip-event EXEC command enables logging of selected SLIP events, such as various types of errors, enabling and disabling of SLIP mode on a line, and so on. The volume of output for this command is much lower than that for the debug slip command.
This section provides an alphabetically arranged summary of the SLIP global configuration commands.
[no] async-bootp tag [:hostname] data...
Specifies extended BootP requests as defined in RFC1084. The argument tag is the item being requested, and is one of the following:
The optional argument :hostname indicates that this entry applies only to the host specified. The argument :hostname accepts both an IP address and logical host name. The argument data can be a list of IP addresses entered in dotted decimal notation or as logical host names, a number, or a quoted string. Use the no async-bootp command to clear the list.
Following is a list of alphabetically arranged SLIP line configuration subcommands. You are also referred to the section "Configuring Asynchronous Lines" in the "System Configuration" chapter for additional information about line configuration.
Cancels SLIP support on the line.
slip access-class number {in|out}
Configures an access list to be used on packets to or from the SLIP host. The argument number is the IP access list number. The keyword in configures a list for packets from the SLIP host; keyword out compares the IP source address against the access list, and only those packets allowed by the access list are transmitted on the asynchronous line.
Specifies the Internet address assigned to the SLIP client at the other end of the serial line connection. The argument internet-address must be on the same network or subnet as the terminal server's network interface.
slip address dynamic [IP-address]
Without an IP address, allows the IP address associated with a SLIP line to be assigned upon access. This subcommand allows flexibility when the lines are in a rotary configuration, when you cannot predict which line a user will be connected to. This feature is supported when a TACACS server is used.
With an IP address, allows a default address to be specified upon access.
Places the line in SLIP mode permanently. The terminal server will not create an EXEC on this line, so it is not available for normal interactive use.
Specifies the limit of the SLIP output queue, which stores packets received from the network waiting to be sent to the SLIP client. The argument packets is the maximum number of packets. Default is two packets.
Allows the line to be used in either SLIP mode or interactive mode. Hanging up the modem or clearing the line puts the line back into interactive mode.
Specifies the size of the largest Internet packet that the SLIP support can handle. The argument bytes is the maximum number of bytes. Default is 1500 bytes
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