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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.
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.
Note 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.
Figure 15-1 illustrates a typical asynchronous SLIP or PPP telecommuting configuration.
Sample SLIP or PPP Telecommuting Configuration
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.
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. Figure 15-2 shows a sample asynchronous routing configuration.
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.
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:
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.
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:
Note In Release 9.1, SLIP was configured and monitored using slip line, EXEC, and debug commands. Beginning with Release 9.21, SLIP and PPP asynchronous interfaces are configured using async commands in interface command mode.
To specify an asynchronous interface, perform the following task in global configuration mode.
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.
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.
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:
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.
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.
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:
Perform the following task in interface configuration mode to assign a permanent default asynchronous 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.
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:
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.
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.
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.
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.
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 IPXPPP on Virtual Asynchronous Interfaces."
To enable IP-SLIP on a synchronous or asynchronous interface, perform the following tasks, beginning in interface configuration mode:
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:
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To enable IP-PPP on a synchronous or asynchronous interface, perform the following tasks, beginning in interface configuration mode:
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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 IPXPPP on Virtual Asynchronous Interfaces" later in this chapter.
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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| 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.
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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| 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.
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.
Note These tasks enable SLIP and PPP on a virtual asynchronous interfaces on a global basis on the access server. To configure SLIP or PPP on a per-VTY basis, use the translate command.
To create a virtual asynchronous interface, perform the following task in global configuration mode:
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:
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.
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 Figure 15-3, 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.
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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| 1 Every loopback interface must have a unique IPX network number. |
To route IP packets using the IGRP, RIP, and OSPF routing protocols on virtual asynchronous interfaces, perform the following task in global configuration mode:
When you make a connection, you must specify the routing keyword on the SLIP or PPP command line.
Note The vty-async dynamic routing command is similar to the async dynamic routing command, except that the async dynamic routing command is used for physical asynchronous interfaces, and the vty-async dynamic-routing command is used on virtual terminal lines configured for asynchronous protocol functionality.
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:
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:
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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.
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:
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:
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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.
You can enable Challenge Handshake Authentication Protocol (CHAP) or Password Authentication Protocol (PAP) for authentication of PPP on virtual asynchronous interfaces.
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:
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.
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:
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:
To configure the access server to allow a PPP or SLIP session to start automatically, perform the following tasks in line configuration mode
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| 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.
Note When using autoselect, the activation character should be set to the default Return, and exec-character-bits to 7. If you change these defaults, the application will not recognize the activation request.
To tune IP performance, complete the tasks in the following sections:
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:
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| 1 This command is documented in the "IP Commands" chapter of the Access and Communication Servers Command Reference publication. |
Note When compression is enabled, fast switching is disabled. Fast processors can handle several fast interfaces, such as T1s, that are running header compression. However, you should think carefully about your network's traffic characteristics before compressing TCP headers. You might want to use the monitoring commands to help compare network utilization before and after enabling header compression.
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:
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| 1 This command is documented in the "IP Commands" chapter of the Access and Communication Servers Command Reference publication. |
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:
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| 1 These commands are documented in the "IP Commands" chapter of the Access and Communication Servers Command Reference publication. |
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:
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| 1 These commands are documented in the "IP Commands" chapter of the Access and Communication Servers Command Reference publication. |
Note This task normally should not be necessary. It should be performed only under the guidance of technical staff. Incorrect configuration can seriously degrade the performance of your router.
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:
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:
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:
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.
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:
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:
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.
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.
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.
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:
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:
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:
To define an access list for packets to the IP host, perform the following task in interface configuration mode:
To configure your access server support to respond to BOOTP requests from client machines, perform the following task in global configuration mode:
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This section describes the following monitoring and maintenance tasks:
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:
To debug PPP links, perform the following tasks in privileged EXEC mode:
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| 1 Refer to the chapter "Configuring Dial-on-Demand Routing" in this publication for more information about the Challenge Handshake Authentication Protocol (CHAP). |
This section contains asynchronous configuration examples. Each configuration is designed to illustrate different communication requirements.
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.
Note The interface number is the same as the absolute line number, in decimal format. The Cisco 2500 defaults to decimal numbers. The ASM-CS displays in octal format. To display line numbers in decimal rather than octal format on the ASM-CS, use the service decimal-tty command. Refer to the chapter "System Management Commands" in the Access and Communication Servers Command Reference publication for a description of the service decimal-tty command.
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.
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.
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.
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.
In 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.
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.
In 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.
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.
In 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.
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.
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.
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.
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.
The 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.
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.
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.
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.
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.
Figure 15-4 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:
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:
Figure 15-5 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:
The configuration is as follows:
If you want to pass IP routing updates across the asynchronous link, issue the following commands:
Next, complete these steps to configure the asynchronous lines between the access servers, starting in global configuration mode:
Finally, configure routing as described in the Router Products Configuration Guide, using one of the following methods. The server can route packets three different ways:
1. Use ARP, which is the default behavior.
2. Use a default-gateway by issuing the command ip default-gateway x.x.x.x, where x.x.x.x is the IP address of a locally attached router.
3. Run an IP routing protocol (RIP, IGRP, EIGRP, or OSPF).
Figure 15-6 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:
The configuration is as follows:
Posted: Thu Aug 21 14:38:16 PDT 2003
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