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Table Of Contents
Cisco's Implementation of SLIP and PPP
Asynchronous Network Connections and Routing
Asynchronous Interfaces and Broadcasts
Telecommuting Configuration Task List
Configure Asynchronous Interfaces
Specify an Asynchronous Interface
Configure SLIP or PPP Encapsulation
Specify Dedicated or Interactive Mode
Configure Dedicated Network Mode
Return a Line to Interactive Mode
Configure the Interface Addressing Method for Remote Devices
Assign a Default Asynchronous Address
Allow an Asynchronous Address to Be Assigned Dynamically
Assign IP Addresses for Local Devices
Configure Transport-Layer Protocols over SLIP and PPP
IPX-PPP—Associating Asynchronous Interfaces with Loopback Interfaces
IPX-PPP—Using Dedicated IPX Network Numbers for Each Interface
Enable SLIP and PPP on Virtual Asynchronous Interfaces
Create Virtual Asynchronous Interfaces
Enable Protocol Translation of SLIP and PPP on Virtual Asynchronous Interfaces
Enable IPX-PPP on Virtual Asynchronous Interfaces
Enable Dynamic Routing on Virtual Asynchronous Interfaces
Enable TCP/IP Header Compression on Virtual Asynchronous Interfaces
Enable Keepalive Updates on Virtual Asynchronous Interfaces
Set an MTU on Virtual Asynchronous Interfaces
Enable PPP Authentication on Virtual Asynchronous Interfaces
Enable PPP Authentication via TACACS on Virtual Asynchronous Interfaces
Configure Automatic Protocol Startup
Configure Performance Parameters
Set the TCP Connection Attempt Time
Control Route Cache Invalidation
Force Header Compression at the EXEC Level
Specify the MTU Size of IP Packets
Improve Asynchronous PPP Performance
Provide Backward Compatibility for SLIP and PPP
Modify the IP Output Queue Size
Configure Support for Extended BOOTP Requests
Monitor and Maintain Asynchronous Interfaces
Asynchronous Interface Configuration Examples
Dedicated Asynchronous Interface Configuration Example
IP-SLIP Example—Asynchronous Interface
IPX-PPP—Loopback Interface Example
IPX-PPP—Using Dedicated IPX Network Numbers for Each Interface
IPX-PPP over X.25 to an IPX Network on VTY lines
Restricted Access on an Asynchronous Interface Example
Asynchronous Routing and Dynamic Addressing Configuration Example
TCP Header Compression Configuration Example
Conserving Network Addresses Using the IP Unnumbered Feature Example
Configuring Routing on a Dedicated Dial-In Router Example
Configuring an Asynchronous Interface as the Only Network Interface Example
Configuring an Interface Example
Remote Network Access Using PPP—A Basic Configuration
Remote Network Access Using PPP—Routing IP
Remote Network Access—Leased Line with Dial-Backup Using PPP
Configuring SLIP and PPP
This chapter describes how to configure asynchronous interfaces for telecommuting applications using Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP) encapsulation. See the Access and Communication Servers Command Reference publication for a complete description of the commands listed in this chapter.
Refer to the Cisco Access Connection Guide for information about EXEC user commands and establishing SLIP and PPP connections.
Cisco's Implementation of SLIP and PPP
SLIP and PPP define methods of sending Internet Protocol (IP) packets over standard RS-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.
The access server concentrates a large number of SLIP or PPP PC or workstation client hosts onto a network interface, allowing the PCs to communicate with any host on the network. The access server can support any combination of SLIP or PPP lines and lines dedicated to normal asynchronous devices such as terminals and modems. Refer to RFC 1055 for more information about SLIP, and RFCs 1331 and 1332 for more information about PPP.
PPP is a newer, more robust protocol than SLIP and it contains protocols that can detect or prevent misconfiguration. SLIP is an older protocol that is supported on more machines.
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Most asynchronous serial links have very low bandwidth. Take care to configure your system so the links will not be overloaded. Consider using default routes and filtering routing updates to prevent them from being sent on these lines.
illustrates a typical asynchronous SLIP or PPP telecommuting configuration.
Figure 15-1
Sample SLIP or PPP Telecommuting Configuration
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Responding to BOOTP Requests
There is an asynchronous BOOTP server in your access server. This means that SLIP and PPP clients can send BOOTP requests to the access server, and the access server will respond with information about the network. For example, the client can send a BOOTP request to find out what its IP address is and where the boot file is located, and the access server can respond with the information.
BOOTP allows a client machine to discover its own IP address, the address of the access server, and the name of a file to be loaded into memory and executed. There are typically two phases to using BOOTP: first, the client's address is determined and the bootfile is selected; then the file is transferred, typically using TFTP.
BOOTP compares to RARP as follows: Reverse Address Resolution Protocol (RARP) is an older protocol that allows a client to determine its IP address if it knows its hardware address. (Refer to the chapters "Configuring IP" and "Configuring IP Routing Protocols," later in this publication, for more information about RARP.) However, RARP is a hardware link protocol, so it can only be implemented on hosts that have special kernel or driver modifications that allow access to these raw packets. BOOTP does not require kernel modifications.
BOOTP supports the extended BOOTP requests specified in RFC 1084 and works for both SLIP and PPP encapsulation.
Asynchronous Network Connections and Routing
Line configuration commands configure a connection to a terminal or a modem. Interface configuration (async) commands described in the "Configuring SLIP and PPP" chapter of this publication configure a line as an asynchronous network interface over which networking functions are performed.
Your access server also supports IP routing connections for communication that requires connecting one network to another.
Beginning with Cisco IOS Release 10.3, your access server supports protocol translation for SLIP and PPP between other network devices running Telnet, LAT, or X.25. The Cisco IOS software also supports translation for byte-oriented traffic to and from a packet assembler/disassembler (PAD). For example, you can send IP packets across a public X.25 PAD network using SLIP or PPP encapsulation when SLIP or PPPprotocol translation is enabled. For more information, refer to the chapter "Configuring Protocol Translation:" in this publication.
If asynchronous dynamic routing is enabled, you can enable routing at the user level by using the routing keyword with the slip or ppp EXEC command.
Asynchronous interfaces offer both dedicated and dynamic address assignment, configurable hold queues and IP packet sizes, extended BOOTP requests, and permit and deny conditions for controlling access to lines. shows a sample asynchronous routing configuration.
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Figure 15-2 Sample Asynchronous Routing Configuration
Asynchronous Interfaces and Broadcasts
Access servers recognize a variety of IP broadcast addresses. When an access server receives an IP packet from an asynchronous client, it rebroadcasts the packet onto the network without changing the IP header. The access server does not alter the packet's broadcast address to match the form of broadcast address it prefers.
The access server receives a copy of asynchronous client broadcasts, and responds to BOOTP requests with the current IP address assigned to the asynchronous interface on which the request was received. This facility allows the asynchronous client software to automatically determine its own IP address.
Telecommuting Configuration Task List
To configure your access server to support telecommuting, you must perform the first task in the following list on your asynchronous interfaces. Perform the rest of the tasks to customize the asynchronous interface for your particular network environment and to monitor asynchronous connections:
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If you want to call back a PPP client requesting asynchrous callback, refer to the section "Call Back PPP Clients" in the "Configuring Terminal Lines and Modem Support" chapter.
The steps to perform these tasks are described in the following sections. See the "Asynchronous Interface Configuration Examples" section at the end of this chapter for examples of asynchronous configuration files. Tasks are performed in global configuration mode unless otherwise specified.
Configure Asynchronous Interfaces
To configure your access server to support telecommuting, configure basic functionality on your asynchronous interfaces, and then customize the interfaces for your environment. Basic configuration tasks include the following:
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Specify an Asynchronous Interface
To specify an asynchronous interface, perform the following task in global configuration mode.
Configure SLIP or PPP Encapsulation
SLIP and PPP are methods of encapsulating datagrams and other network-layer protocol information over point-to-point links. To configure the default encapsulation on an asynchronous interface, perform the following task in interface configuration mode.
Configure PPP or SLIP encapsulation on an asynchronous line.
encapsulation {ppp | slip}
In order to use SLIP or PPP, the access 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, but you must configure it manually if you configure SLIP or PPP via the interface async command.
When an asynchronous interface is encapsulated with PPP or SLIP, IP fast switching is enabled. For more information on IP fast switching, refer to the "Enable Fast Switching" section later in this chapter.
Specify Dedicated or Interactive Mode
You can configure one or more asynchronous interfaces on your access server to be in dedicated network interface mode. In dedicated mode, an interface is automatically configured for SLIP or PPP connections. There is no user prompt or EXEC level, and no end-user commands are required to initiate telecommuting connections. If you want a line to be used only for SLIP or PPP connections, configure the line for dedicated mode.
In interactive mode, a line can be used to make any type of connection, depending on the EXEC command entered by the user. For example, depending on its configuration, the line could be used for Telnet or XRemote connections, or SLIP or PPP encapsulation. The user is prompted for an EXEC command before a connection is initiated.
This section describes the following tasks:
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Configure Dedicated Network Mode
You can configure an asynchronous interface to be in dedicated network mode. When the interface is configured for dedicated mode, the end user cannot change the encapsulation method, address, or other parameters.
To configure an interface for dedicated network mode, perform the following task in interface configuration mode.
Refer to the chapter "Managing the System," earlier in this publication, for more information about automatic dialing using DTR.
Return a Line to Interactive Mode
After a line has been placed in dedicated mode, perform the following task in interface configuration mode to return it to interactive mode.
By default, no asynchronous mode is configured. In this state, the line is not available for inbound networking because the SLIP and PPP connections are disabled.
Configure the Interface Addressing Method for Remote Devices
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 access 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 users 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.
This section describes the following tasks:
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Assign a Default Asynchronous Address
Perform the following task in interface configuration mode to assign a permanent default asynchronous address:
Assign a default IP address to an asynchronous interface.
async default ip address address
Use the no form of this command to disable the default address. If the server has been configured to authenticate asynchronous connections, you are prompted for a password after entering the SLIP or PPP EXEC command before the line is placed into asynchronous mode.
The assigned default address is implemented when the user enters the slip default or ppp default EXEC command. The transaction is validated by the TACACS server (when enabled) and the line is put into network mode using the address that is in the configuration file.
Configuring a default address is useful when the user is not required to know the IP address to gain access to a system; for example, users of a server that is available to many students on a campus. Instead of requiring each user to know an IP address, they need only enter the slip default or ppp default EXEC command and let the server select the address to use. See the Cisco Access Connection Guide for more information about the slip and ppp EXEC commands.
Allow an Asynchronous Address to Be Assigned Dynamically
When a line is configured for dynamic assignment of asynchronous addresses, the user enters the slip or ppp EXEC command and is prompted for an address or logical host name. The address is validated by the Terminal Access Controller Access System (TACACS), when enabled, and the line is assigned the given address and put into asynchronous mode. Assigning asynchronous addresses dynamically is also useful when you want to assign set addresses to users. For example, an application on a personal computer that automatically dials in using SLIP and polls for electronic mail messages can be set up to dial in periodically and enter the required IP address and password.
To assign asynchronous addresses dynamically, perform the following task in interface configuration mode:
Allow the IP address to be assigned when the protocol is initiated.
async dynamic address
The dynamic addressing features of the internetwork allow packets to get to their destination and back regardless of the access server or network they are sent from. For example, if a host such as a laptop computer moves from place to place it can keep the same address no matter where it is dialing in from.
Logical host names are first converted to uppercase and then sent to the TACACS server for authentication.
Assign IP Addresses for Local Devices
The local address is set using the ip address or ip unnumbered command.
IP addresses identify locations to which IP datagrams can be sent. You must assign each router interface an IP address. See the Internetworking Technology Overview publication for detailed information on IP addresses.
To assign an IP address to a network interface on the access server, perform the following task in interface configuration mode:
A subnet mask identifies the subnet field of a network address. Subnets are described in the Internetworking Technology Overview publication.
Conserve Network Addresses
When asynchronous routing is enabled, you might find it necessary to conserve network addresses by configuring the asynchronous interfaces as unnumbered. An unnumbered interface does not have an address. Network resources are therefore conserved because fewer network numbers are used and routing tables are smaller.
To configure an unnumbered interface, perform the following task in interface configuration mode.
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.
You can use the IP unnumbered feature on the access server whether or not the system on the other end of the asynchronous link supports this feature. The IP unnumbered feature is transparent to the other end of the link because each system bases its routing activities on information in the routing updates it receives and on its own interface address on the link.
Enable Asynchronous Routing
To route IP packets, perform the following task in interface configuration mode to enable routing protocols IGRP, RIP, and OSPF, on an interface.
When the user makes a connection, they must specify /routing on the SLIP or PPP command line.
Configure Transport-Layer Protocols over SLIP and PPP
You can configure transport-layer protocols, such as AppleTalk, IP, and IPX, over SLIP and PPP. SLIP supports only IP, while PPP supports each of these protocols. Refer to the sections that follow to configure these protocols over SLIP and PPP.
You can also configure IPX-PPP on VTYs. Refer to the section "Enable IPX-PPP on Virtual Asynchronous Interfaces."
Configure IP-SLIP
To enable IP-SLIP on a synchronous or asynchronous interface, perform the following tasks, beginning in interface configuration mode:
Configure AppleTalk-PPP
You can configure an asynchronous interface on the access server so that users can access AppleTalk zones by dialing into the access server via PPP to this interface. Users accessing the network can run AppleTalk and IP natively on a remote Macintosh, access any available AppleTalk zones from Chooser, use networked peripherals, and share files with other Macintosh users.
You create a virtual network that exists only for accessing an AppleTalk internet through the server. To create a new AppleTalk zone, issue the appletalk virtual-net command and use a new zone name; this network number is then the only one associated with this zone. To add network numbers to an existing AppleTalk zone, use this existing zone name in the command; this network number is then added to the existing zone.
Routing is not supported on these interfaces.
To enable ATCP for PPP, perform the following tasks in interface configuration (asynchronous) mode:
Configure IP-PPP
To enable IP-PPP on a synchronous or asynchronous interface, perform the following tasks, beginning in interface configuration mode:
Configure IPX-PPP
You can configure IPX to run over PPP on synchronous serial and asynchronous serial interfaces using one of two methods.
The first method associates an asynchronous interface with a loopback interface configured to run IPX. It permits you to configure IPX-PPP on asynchronous interfaces only.
The second method permits you to configure IPX/PPP on asynchronous and synchronous serial interfaces. However, it requires that you specify a dedicated IPX network number for each interface, which can require a substantial number of network numbers for a large number of interfaces.
You can also configure IPX to run on VTYs configured for PPP . Refer to the section "Enable IPX-PPP on Virtual Asynchronous Interfaces" later in this chapter.
IPX-PPP—Associating Asynchronous Interfaces with Loopback Interfaces
To permit IPX client connections to an asynchronous interface, the interface must be associated with a loopback interface configured to run IPX. To permit such connections, perform the following tasks, beginning in global configuration mode:
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ipx routing [node]
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interface loopback number
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ipx network network1
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exit
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interface async number
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ip unnumbered type number
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encapsulation ppp
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async mode interactive
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ipx ppp-client Loopback number
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ipx sap-interval 0
1 Every interface must have a unique IPX network number.
If you are configuring IPX-PPP on asynchronous interfaces, you should filter routing updates on the interface. Most asynchronous serial links have very low bandwidth, and routing updates take up a great deal of bandwidth. To filter routing updates, refer to the section "Create Filters for Updating the Routing Table" in the "Configuring Novell IPX" chapter of this publication.
IPX-PPP—Using Dedicated IPX Network Numbers for Each Interface
To enable IPX-PPP, perform the following tasks starting in global configuration mode. The first five tasks are required. The last task is optional:
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ipx routing [node]
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interface type number
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encapsulation ppp
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async mode interactive
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ipx network network1
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ipx sap-interval 0
1 Every interface must have a unique ipx network number.
If you are configuring IPX-PPP on asynchronous interfaces, you should filter routing updates on the interface. Most asynchronous serial links have very low bandwidth, and routing updates take up a great deal of bandwidth. To filter routing updates, refer to the section "Create Filters for Updating the Routing Table" in the "Configuring Novell IPX" chapter of this publication.
Enable SLIP and PPP on Virtual Asynchronous Interfaces
The Cisco IOS software permits you to configure asynchronous protocol features, such as SLIP and PPP, on virtual terminal (VTY) lines. SLIP and PPP normally function only on asynchronous interfaces, and not on VTY lines. When you configure a virtual terminal line to support asynchronous protocol features, you are creating virtual asynchronous interfaces on the VTY lines. One practical benefit of virtual asynchronous interfaces is the ability to tunnel SLIP and PPP over X.25, TCP, or LAT on VTY lines to an IP or IPX network. You tunnel SLIP and PPP using the protocol translation facility. For more information, refer to the chapter "Configuring Protocol Translation" in this publication.
Perform the tasks in the following sections to configure and use virtual asynchronous interfaces. The first task is required; the remaining tasks are optional.
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Create Virtual Asynchronous Interfaces
To create a virtual asynchronous interface, perform the following task in global configuration mode:
Configure all virtual terminal lines to support asynchronous protocol features.
vty-async
Enable Protocol Translation of SLIP and PPP on Virtual Asynchronous Interfaces
One practical benefit of enabling virtual asynchronous interfaces is the ability to tunnel SLIP and PPP over X.25, thus extending remote node capability into the X.25 area. You can also tunnel SLIP and PPP over Telnet or LAT on virtual terminal lines. You can tunnel SLIP and PPP over X.25, LAT, or Telnet, but you do so by using the protocol translation feature in the Cisco IOS software. Refer to the "Configuring Protocol Translation" chapter in this publication for more information about protocol translation.
To tunnel incoming dial-up SLIP or PPP connections over X.25, LAT, or TCP to an IP network, you can use one-step protocol translation or two-step protocol translation, as follows:
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To make a connection to a network device using any supported protocol, refer to the Cisco Access Connection Guide.
For an example of tunneling SLIP across an X.25 PAD WAN, refer to the "Configuring Protocol Translation" chapter in this publication.
Enable IPX-PPP on Virtual Asynchronous Interfaces
You can enable IPX-PPP on virtual terminal lines (VTYs), which permits clients to log into a VTY on an access server, invoke a PPP session at the EXEC prompt to a host, and run IPX to the host.
For example, in , the client Terminal1 on the X.25 network logs into the VTY on Access Server1, which is configured for IPX-PPP. When the user connects to the access server and the EXEC prompt appears, the user issues the PPP command to connect to the IPX host. The VTY is configured to run IPX, so when the PPP session is established from the access server. Terminal1 can access the IPX host using an IPX application.
Figure 15-3 IPX-PPP on a Virtual Asynchronous Interface
To enable IPX to run over your PPP sessions on VTY lines, perform the following tasks, beginning in global configuration mode:
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ipx routing [node]
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interface loopback number
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ipx network network1
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vty-async ipx ppp-client Loopback number
1 Every loopback interface must have a unique IPX network number.
Enable Dynamic Routing on Virtual Asynchronous Interfaces
To route IP packets using the IGRP, RIP, and OSPF routing protocols on virtual asynchronous interfaces, perform the following task in global configuration mode:
Enable dynamic routing of IP packets on all virtual terminal lines.
vty-async dynamic-routing
When you make a connection, you must specify the routing keyword on the SLIP or PPP command line.
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Enable TCP/IP Header Compression on Virtual Asynchronous Interfaces
You can compress the headers on TCP/IP packets on virtual asynchronous interfaces to reduce their size and increase performance. This feature only compresses the TCP header, so it has no effect on UDP packets or other protocol headers. The TCP header compression technique, described fully in RFC 1144, is supported on virtual asynchronous interfaces using SLIP and PPP encapsulation. You must enable compression on both ends of the connection.
You can optionally specify outgoing packets to be compressed only if TCP incoming packets on the same virtual terminal line are compressed. If you do not specify this option, the access server will compress all traffic. The default is no compression. This option is valid for SLIP.
To compress the headers of outgoing TCP packets on virtual asynchronous interfaces, perform the following task in global configuration mode:
Enable header compression on IP packets on all virtual terminal lines.
vty-async header-compression [passive]
Enable Keepalive Updates on Virtual Asynchronous Interfaces
Keepalives are enabled on all virtual asynchronous interfaces by default. To change the keepalive timer or disable it on virtual asynchronous interfaces, perform the following task in global configuration mode:
Specify the frequency with which the Cisco IOS software sends keepalive messages to the other end of an asynchronous serial link.
vty-async keepalive [seconds]
The default interval is 10 seconds. It is adjustable in one-second increments from 0 to 32,767 seconds. To turn off keepalive updates, set the value to 0. A connection is declared down after three update intervals have passed without receiving a keepalive packet.
Virtual terminal lines have very low bandwidth. When adjusting the keepalive timer, large packets can delay the smaller keepalive packets long enough to cause the session to disconnect. You might need to experiment to determine the best value.
Set an MTU on Virtual Asynchronous Interfaces
The maximum transmission unit (MTU) refers to the size of an IP packet. You might want to change to a smaller MTU size for IP packets transmitted on a virtual asynchronous interface for any of the following reasons:
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For example, at 9600 baud a 1500 byte packet takes about 1.5 seconds to transmit. This delay would indicate that you want an MTU size of about 200 (1.5 seconds / 0.2 seconds = 7.5 and 1500 byte packet/ 7.5 = 200 byte packet).
To specify the maximum IP packet size, perform the following task in interface configuration mode:
Specify the size of the largest IP packet that the virtual asynchronous interface can support.
vty-async mtu bytes
The default MTU size is 1500 bytes. Possible values are 64 bytes to 1,000,000 bytes.
TCP running on the device to which the access server is connected can have a different MTU size than what is configured on the access server. Because the access server performs IP fragmentation of packets larger than the specified MTU. Do not change the MTU size unless the SLIP or PPP implementation running on the host at the other end of the asynchronous line supports reassembly of IP fragments.
Enable PPP Authentication on Virtual Asynchronous Interfaces
You can enable Challenge Handshake Authentication Protocol (CHAP) or Password Authentication Protocol (PAP) for authentication of PPP on virtual asynchronous interfaces.
Enable CHAP
Access control using Challenge Handshake Authentication Protocol (CHAP) is available on all virtual asynchronous interfaces configured for PPP encapsulation. The authentication feature reduces the risk of security violations on your access server.
When CHAP is enabled, a remote device (a PC, workstation, or access server) attempting to connect to the local access server is requested, or "challenged," to respond.
The challenge consists of an ID, a random number, and either the host name of the local access server or the name of the user on the remote device. This challenge is transmitted to the remote device.
The required response consists of two parts:
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When the local access server receives the challenge response, it verifies the secret by looking up the name given in the response and performing the same encryption operation. The secret passwords must be identical on the remote device and the local access server.
By transmitting this response, the secret is never transmitted, thus preventing other devices from stealing it and gaining illegal access to the system. Without the proper response, the remote device cannot connect to the local access server.
CHAP transactions occur only when a link is established. The local access server does not request a password during the rest of the session. (The local access server can, however, respond to such requests from other devices during a session.)
To use CHAP on virtual asynchronous interfaces for PPP, perform the following task in global configuration mode:
CHAP is specified in RFC 1334. It is an additional authentication phase of the PPP Link Control Protocol.
Once you have enabled CHAP, the local access server requires a response from the remote devices. If the remote device does not support CHAP, no traffic is passed to that device.
Enable PAP
Access control using the Password Authentication Protocol (PAP) is available on all virtual asynchronous interfaces configured for PPP encapsulation. The authentication feature reduces the risk of security violations on your access server.
To use PAP, perform the following task in interface configuration mode:
Enable PPP Authentication via TACACS on Virtual Asynchronous Interfaces
Access control using the Terminal Access Controller Access Control System (TACACS) is available on all virtual asynchronous interfaces configured for PPP encapsulation. The authentication feature reduces the risk of security violations on your access server.
To use TACACS with either CHAP or PAP, perform the following task in global configuration mode:
Configure Automatic Protocol Startup
To configure the access server to allow a PPP or SLIP session to start automatically, perform the following tasks in line configuration mode
Configure a line to automatically start an ARA, PPP or SLIP session.
autoselect {arap | ppp | slip} | during login1
1 This command is documented in the "Terminal Line and Modem Support Commands" chapter of the Access and Communication Servers Command Reference publication.
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The autoselect command permits the access server to allow an appropriate process to start automatically when a starting character is received. The access server detects either a Return character, which is the start character for an EXEC session, or the start character for the ARA protocol. By using the optional during login argument, the username or password prompt is displayed without pressing the Return key. While the Username or Password name is presented, you can choose to answer these prompts or to start sending packets from an autoselected protocol. Refer to the end of this chapter for configuration examples.
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Configure Performance Parameters
To tune IP performance, complete the tasks in the following sections:
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Compress TCP Packet Headers
You can compress the headers of your TCP/IP packets in order to reduce their size, thereby increasing performance. Header compression is particularly useful on networks with a large percentage of small packets, such as those supporting many Telnet connections. This feature only compresses the TCP header, so it has no effect on UDP packets or other protocol headers. The TCP header compression technique, described fully in RFC 1144, is supported on serial lines using HDLC or PPP encapsulation. You must enable compression on both ends of a serial connection.
You can optionally specify outgoing packets to be compressed only if TCP incoming packets on the same interface are compressed. If you do not specify this option, the access server will compress all traffic. The default is no compression.
You can also specify the total number of header compression connections that can exist on an interface. You should configure one connection for each TCP connection through the specified interface.
To enable compression, perform either of the following optional tasks in interface configuration mode:
Enable TCP header compression.
ip tcp header-compression [passive]
Specify the total number of header compression connections that can exist on an interface.
ip tcp compression-connections number1
1 This command is documented in the "IP Commands" chapter of the Access and Communication Servers Command Reference publication.
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Set the TCP Connection Attempt Time
You can set the amount of time the access server will wait to attempt to establish a TCP connection. In previous versions of the Cisco IOS software, the system would wait a fixed 30 seconds when attempting to do so. This amount of time is not sufficient in networks that have dial-up asynchronous connections, such as a network consisting of dial-on-demand links that are implemented over modems, because it will affect your ability to Telnet over the link (from the access server) if the link must be brought up.
Because the connection attempt time is a host parameter, it does not pertain to traffic going through the access server, just to traffic originated at the access server.
To set the TCP connection attempt time, perform the following task in global configuration mode:
Set the amount of time the access server will wait to attempt to establish a TCP connection.
ip tcp synwait-time seconds1
1 This command is documented in the "IP Commands" chapter of the Access and Communication Servers Command Reference publication.
Enable Fast Switching
Fast switching involves the use of a high-speed switching cache for IP routing. With fast switching, destination IP addresses are stored in the high-speed cache so that some time-consuming table lookups do not need to be done. Our access servers generally offer better packet transfer performance when fast switching is enabled.
To enable or disable fast switching, perform the following tasks in interface configuration mode:
Enable fast-switching (use of a high-speed route cache for IP routing).
ip route-cache1
Disable fast switching and enable load balancing on a per-packet basis.
no ip route-cache1
1 These commands are documented in the "IP Commands" chapter of the Access and Communication Servers Command Reference publication.
Control Route Cache Invalidation
The high-speed route cache used by IP fast switching is invalidated when the IP routing table changes. By default, the invalidation of the cache is delayed slightly to avoid excessive CPU load while the routing table is changing.
To control route cache invalidation, perform the following tasks in global configuration mode as needed for your network:
Allow immediate invalidation of the cache.
no ip cache-invalidate-delay1
Delay invalidation of the cache.
ip cache-invalidate-delay [minimum maximum quiet threshold]1
1 These commands are documented in the "IP Commands" chapter of the Access and Communication Servers Command Reference publication.
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Optimize Available Bandwidth
Asynchronous lines have relatively low bandwidth and can easily be overloaded, resulting in slow traffic across these lines.
To optimize available bandwidth, perform any of the following tasks:
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Configure Header Compression
One way to optimize available bandwidth is by using TCP header compression. Van Jacobson TCP header compression (defined by RFC 1144) can increase bandwidth availability between two and five times when compared to lines not using header compression. Theoretically, it can improve bandwidth availability by a ratio of seven to one.
To configure header compression, perform the following task in interface configuration mode:
Configure Van Jacobson TCP header compression on the asynchronous link.
ip tcp header-compression [on | off | passive]
Force Header Compression at the EXEC Level
On SLIP interfaces, you can force header compression at the EXEC prompt on a line on which header compression has been set to passive. This allows more efficient use of the available bandwidth and does not require entering privileged configuration mode.
To implement header compression, perform the following task in interface configuration mode:
Allow status of header compression to be assigned at the user level.
ip tcp header compression passive
For PPP interfaces, the passive option functions the same as the on option.
See the Cisco Access Connection Guide for information about the slip and ppp EXEC commands. You cannot force header compression if header compression on the asynchronous interface is off.
Specify the MTU Size of IP Packets
The maximum transmission unit (MTU) refers to the size of an IP packet. You might want to change to a smaller MTU size for any of the following reasons:
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For example, at 9600 baud a 1500 byte packet takes about 1.5 seconds to transmit. This delay would indicate that you want an MTU size of about 200 (1.5 seconds / 0.2 seconds = 7.5 and 1500 byte packet/ 7.5 = 200 byte packet).
To specify maximum IP packet size, perform the following task in interface configuration mode:
Specify the size of the largest IP packet that the asynchronous line can support.
ip mtu bytes
The MTU size can be negotiated by TCP, regardless of the asynchronous interface settings. In other words, TCP running on the device to which the access server is connected can negotiate for a different MTU size than is configured on the access server. The access server performs IP fragmentation of packets larger than the specified MTU. Do not change the MTU size unless the SLIP or PPP implementation running on the host at the other end of the asynchronous line supports reassembly of IP fragments. Because each fragment occupies a spot in the output queue, it might also be necessary to increase the size of the SLIP or PPP hold queue, if your MTU size is such that you might have a high amount of fragments of packets in the output queue.
Improve Asynchronous PPP Performance
To improve asynchronous PPP performance, use the ppp accm interface command. In cases where devices have minimal PPP stacks that do not negotiate PPP Asynchronous Control Character Maps (ACCM), the RFC standard default of 0xffffffff is often used, resulting in poor performance.
The ppp accm command allows you to set the initial values used by the access during LCP negotiations with a peer device to alleviate this problem.
Provide Backward Compatibility for SLIP and PPP
To provide backward compatibility for client software scripts expecting SLIP and PPP dialog to be formatted with software release 9.1 or earlier, use the service old-slip-prompts global configuration command. You can format SLIP and PPP transmition by performing the following task in global configuration mode.
Modify the IP Output Queue Size
The IP output queue stores packets received from the network that are waiting to be sent to the asynchronous client. You can limit the size of the IP output queue to enhance performance by performing the following task in interface configuration mode:
Specify IP Access Lists
Access lists allow the system administrator to control the hosts that a PC can access through an access server. Separate access lists can be defined for asynchronous and for other connections.
The tasks described in this section are as follows:
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Refer to the chapter "Configuring IP," later in this publication, for information about defining IP access lists.
To define an access list for packets from the IP host, perform the following task in interface configuration mode:
Configure an access list for packets from the IP host.
ip access-group access-list-number in
To define an access list for packets to the IP host, perform the following task in interface configuration mode:
Configure an access list for packets being sent to the IP host.
ip access-group access-list-number out
Configure Support for Extended BOOTP Requests
To configure your access server support to respond to BOOTP requests from client machines, perform the following task in global configuration mode:
Specify the access server network information that will be sent in response to BOOTP requests.
async-bootp tag [:hostname] data
Monitor and Maintain Asynchronous Interfaces
This section describes the following monitoring and maintenance tasks:
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To monitor and maintain asynchronous activity, perform one or more of the following tasks in privileged EXEC mode:
To debug asynchronous interfaces, perform the following task in privileged EXEC mode:
Displays errors, changes in interface state, and log input and output.
debug async {framing | state | packets}
To debug PPP links, perform the following tasks in privileged EXEC mode:
Enable debugging of PPP protocol negotiation process.
debug ppp negotiation
Display PPP protocol errors.
debug ppp error
Display PPP packets sent and received.
debug ppp packet
Display errors encountered during remote or local system authentication.1
debug ppp chap
1 Refer to the chapter "Configuring Dial-on-Demand Routing" in this publication for more information about the Challenge Handshake Authentication Protocol (CHAP).
Asynchronous Interface Configuration Examples
This section contains asynchronous configuration examples. Each configuration is designed to illustrate different communication requirements.
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Dedicated Asynchronous Interface Configuration Example
The following example assigns an IP address to an asynchronous interface and places the line in dedicated network mode. Setting the stop bit to 1 is a performance enhancement.
line 20location Department PC Labstopbits 1speed 19200!interface async 20async default ip address 182.32.7.51async mode dedicated
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IP-SLIP Example—Asynchronous Interface
The following example configures IP-SLIP on asynchronous interface 6. The IP address for the interface is assigned to Ethernet 0, interactive mode has been enabled, and the IP address of the client PC running SLIP has been specified.
IP and the appropriate IP routing protocols have already been enabled on the server.
Router(config)# interface async 6Router(config-if)# ip unnumbered ethernet 0Router(config-if)# encapsulation slipRouter(config-if)# async mode interactiveRouter(config-if)# async default ip address 172.18.1.128AppleTalk-PPP Example
The following example configures asynchronous interface 4 on the access server so that users can access AppleTalk zones by dialing into the access server via PPP to this interface. Users accessing the network can run AppleTalk and IP natively on a remote Macintosh, access any available AppleTalk zones from Chooser, use networked peripherals, and share files with other Macintosh users. Routing is not supported on the asynchronous interface 4.
Router(config)# interface async 6Router(config-if)# encapsulation pppRouter(config-if)# appletalk virtual-net 12345 saiviteRouter(config-if)# appletalk client-modeIP-PPP Example
The following example configures IP-PPP on asynchronous interface 6. The IP address for the interface is assigned to Ethernet 0, interactive mode has been enabled, and the IP address of the client PC running PPP has been specified.
IP and the appropriate IP routing protocols have already been enabled on the server.
Router(config)# interface async 6Router(config-if)# ip unnumbered ethernet 0Router(config-if)# encapsulation pppRouter(config-if)# async mode interactiveRouter(config-if)# async default ip address 172.18.1.128IPX-PPP—Loopback Interface Example
The following example shows the process of configuring IPX to run over PPP on an asynchronous interface. The asynchronous interface is associated with a loopback interface configured to run IPX. This example enables a non-routing IPX client to connect to the access server.
Router(config)# ipx routing 0000.0c07.b509Router(config)# interface loopback0Router(config-if)# no ip addressRouter(config-if)# ipx network 544Router(config-if)# ix sap-interval 2000Router(config-if)# exitRouter(config)# interface ethernet0Router(config-if)# ip address 172.21.14.64Router(config-if)# ipx network AC150E00Router(config-if)# ipx encapsulation SAPRouter(config-if)# exitRouter(config)# interface async 3Router(config-if)# ip unnumbered ethernet0Router(config-if)# encapsulation pppRouter(config-if)# async mode interactiveRouter(config-if)# async default ip address 172.18.1.128Router(config-if)# ipx ppp-client loopback0Router(config-if)# ipx sap-interval 0In this example, IPX client connections are permitted to asynchronous interface 3, which is associated with loopback interface 0. Loopback interface 0 is configured to run IPX. Routing updates have been filtered on asynchronous interface 3. Routing updates take up a great deal of bandwidth, and asynchronous interfaces have low bandwidth.
IPX-PPP—Using Dedicated IPX Network Numbers for Each Interface
The following example shows the process of configuring IPX to run over PPP on an asynchronous interface. A dedicated IPX network number has been specified for each interface, which can require a substantial number of network numbers for a large number of interfaces. This example permits an IPX client with routing enabled to connect with the access server.
Router(config)# ipx routing 0000.0c07.b509Router(config)# interface async 6Router(config-if)# ip unnumbered ethernet0Router(config-if)# encapsulation pppRouter(config-if)# async mode interactiveRouter(config-if)# ipx network AC150E00Router(config-if)# ipx sap-interval 0In this example, IPX client connections are permitted to asynchronous interface 6, which has a unique IPX network number. Routing updates have been filtered on asynchronous interface 6. Routing updates take up a great deal of bandwidth, and asynchronous interfaces have low bandwidth.
IPX-PPP over X.25 to an IPX Network on VTY lines
The following example shows the process of enabling IPX-PPP on VTY lines. First, you enable PPP to run on VTY lines, then you associate the VTY line with a loopback interface configured to run IPX. This example enables a non-routing IPX client to connect to the access server.
Router(config)# ipx routing 0000.0c07.b509Router(config)# interface loopback0Router(config-if)# no ip addressRouter(config-if)# ipx network 544Router(config-if)# exitRouter(config-if)# vty-async ipx ppp-client loopback0In this example, IPX client connections are permitted to VTY lines, which have been associated with loopback interface 0. Loopback interface 0 is configured with an IPX network number that is used by the VTY lines.
Restricted Access on an Asynchronous Interface Example
The following example assumes that users are restricted to certain servers designated as asynchronous servers, but that normal terminal users can access anything on the local network.
! access list for normal connectionsaccess-list 1 permit 172.16.0.0 0.0.255.255!access-list 2 permit 172.16.42.55access-list 2 permit 172.16.111.1access-list 2 permit 172.16.55.99!interface async 6async dynamic addressip access-group 1 outip access-group 2 inAsynchronous Routing and Dynamic Addressing Configuration Example
The following example shows a simple configuration that allows routing and dynamic
addressing. With this configuration, if the user specifies /routing in the EXEC slip or ppp command, routing protocols will be sent and received.interface async 6async dynamic routingasync dynamic addressTCP Header Compression Configuration Example
The following example configures async interface 7 with a default IP address, allowing header compression if it is specified in the slip or ppp connection command entered by the user or if the connecting system sends compressed packets.
interface async 7ip address 172.31.79.1async default ip address 172.31.79.2ip tcp header-compression passiveConserving Network Addresses Using the IP Unnumbered Feature Example
The following example shows how to configure your access server for routing using unnumbered interfaces. The source (local) address is shared between Ethernet 0 and async 6 (172.18.1.1). The default remote address is 172.18.1.2.
interface ethernet 0ip address 172.18.1.1 255.255.255.0!interface async 6ip unnumbered ethernet 0async dynamic routing! default address is on the local subnetasync dynamic addressasync default ip address 172.18.1.2ip tcp header-compression passiveThe following example shows how the IP unnumbered configuration works. Although the user assigned an address, the system response shows the interface as unnumbered, and the address typed by the user will be used only in response to BOOTP requests.
Router> slip /compressed 10.11.11.254Password:Entering async mode.Interface IP address is unnumbered, MTU is 1500 bytes.Header compression is On.Configuring Routing on a Dedicated Dial-In Router Example
In the following example, the access server is set up as a dedicated dial-in router. Interfaces are configured as IP unnumbered to conserve network resources, primarily IP addresses.
ip routinginterface ether 0ip address 10.129.128.2 255.255.255.0!interface async 1ip unnumbered ethernet 0async dynamic routing! The addresses assigned with SLIP or PPP EXEC commands are not used except! to reply to BOOTP requests. ! Normally, the routers dialing in will have their own address ! and not use BOOTP at all.async default ip address 10.11.11.254!interface async 2ip unnumbered ethernet 0async default ip address 10.11.12.16ip tcp header-compression passiveasync mode dedicated!! run RIP on the asynchronous lines, because few implementations of SLIP ! understand IGRP. Run IGRP on the ethernet (and in the local network).!router igrp 110network 10.11.12.0! send routes from the asynchronous lines on the production network.redistribute RIP! don't send IGRP updates on the async interfacespassive-interface async 1!router RIPnetwork 10.11.12.0redistribute igrppassive-interface ethernet 0! consider filtering everything except a default route from the routing! updates sent on the (slow) asynchronous linesdistribute-list 1 outip unnumbered async 2async dynamic routingConfiguring an Asynchronous Interface as the Only Network Interface Example
In the following example, one of the asynchronous lines is used as the only network interface. The access server is used primarily as a terminal server, but is at a remote location and dials into the central site for its only network connection.
ip default-gateway 10.11.12.2interface ethernet 0shutdowninterface async 1async dynamic routingip tcp header-compression onasync default ip address 10.11.16.12async mode dedicatedip address 10.11.12.32 255.255.255.0Configuring IGRP Example
In the following example, only the IGRP TCP/IP routing protocol is running; it is assumed that the systems that are dialing in to use routing will either support IGRP or have some other method (for example, a static default route) of determining that the access server is the best place to send most of its packets.
router igrp 111network 10.11.12.0interface ethernet 0ip address 10.11.12.92 255.255.255.0!interface async 1async default ip address 10.11.12.96async dynamic routingip tcp header-compression passiveip unnumbered ethernet 0line 1modem ri-is-cdConfiguring an Interface Example
The following configuration shows interface and line configuration. The interface is configured with access lists, passive header compression and a default address. The line is configured for TACACS authentication.
interface async 1ip access-group 1 inip access-group 1 outip tcp header-compression passiveasync default ip address 172.31.176.201line 1login tacacslocation 457-5xxxexec-timeout 20 0password XXXXXXXXsession-timeout 20stopbits 1Remote Network Access Using PPP—A Basic Configuration
illustrates a simple network configuration comprised of remote PCs with modems connected via modem to an access server. The cloud is a public switched telephone network (PSTN). The modems are connected via asynchronous lines, and the access server is connected to a local network.
In this configuration you will need to configure the following:
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Figure 15-4 Remote Network Access Using PPP
This default address indicates the address of the remote PC to the server, unless the user explicitly specifies another when starting the PPP session.
The server is configured for interactive mode with autoselect enabled, which allows the user to automatically begin a PPP session upon detection of a PPP packet from the remote PC; or, the remote PC can explicitly begin a PPP session by typing PPP at the prompt.
The configuration is as follows:
ip routing!int ethernet 0ip address 192.168.32.12 255.255.255.0!interface async 1encapsulation pppasync mode interactiveasync default ip address 192.168.32.51async dynamic addressip unnumbered ethernet 0line 1autoselect pppmodem callinspeed 19200Remote Network Access Using PPP—Routing IP
illustrates a network configuration that provides routing functionality, allowing routing updates to be passed across the asynchronous lines.
This network is comprised of remote and local PCs connected via modem and network connections to an access server. This access server is connected to a second access server via an asynchronous line running TCP/IP. The second access server is connected to a local network via modem.
For this scenario, you will need to configure the following:
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Figure 15-5 Routing on an Asynchronous Line Using PPP
The configuration is as follows:
interface async 1encapsulation pppasync mode interactiveasync default ip address 192.168.32.10async dynamic addressip unnumbered ethernet 0async dynamic routingIf you want to pass IP routing updates across the asynchronous link, issue the following commands:
line 1autoselect pppmodem callinspeed 19200Next, complete these steps to configure the asynchronous lines between the access servers, starting in global configuration mode:
interface async 2async default ip address 192.168.32.55ip tcp header compression passiveFinally, configure routing as described in the Router Products Configuration Guide, using one of the following methods. The server can route packets three different ways:
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Remote Network Access—Leased Line with Dial-Backup Using PPP
illustrates a scenario where two networks are connected via access servers on a leased line. Redundancy is provided by a dial-backup line over the public switched telephone network so that if the primary leased line goes down, the dial-backup line will be automatically brought up to restore the connection. This configuration would be useful for using an auxiliary port as the backup port for a synchronous port.
In this scenario, you will need to configure the following:
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Figure 15-6 Asynchronous Leased Line with Backup
The configuration is as follows:
hostname routerA!username routerB password ciscochat-script backup "" "AT" TIMEOUT 30 OK atdt\T TIMEOUT 30 CONNECT \c !interface Serial0backup interface Async1ip address 192.168.222.12 255.255.255.0!interface Async1ip address 172.16.199.1 255.255.255.0encapsulation pppasync default ip address 172.16.199.2async dynamic addressasync dynamic routingasync mode dedicateddialer in-banddialer map IP 172.16.199.2 name routerB modem-script backup broadcast 3241129 dialer-group 1ppp authentication chap!dialer-list 1 protocol ip permit!line aux 0modem InOutrxspeed 38400txspeed 38400
