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Configuring X.25 and LAPB


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Configuring X.25 and LAPB



LAPB Configuration Task List



Configure a LAPB Datagram Transport



Modify LAPB Protocol Parameters



Configure Priority and Custom Queuing for LAPB



Configure Transparent Bridging over Multiprotocol LAPB



X.25 Configuration Task List



Configure an X.25 Interface



Configure Additional X.25 Interface Parameters



Configure an X.25 Datagram Transport



Configure Additional X.25 Datagram Transport Features



Configure Priority Queuing or Custom Queuing for X.25



Configure X.25 Routing



Configure Additional X.25 Routing Features



Configure CMNS Routing



Configure DDN or BFE X.25



Create X.29 Access Lists



Create an X.29 Profile Script



Monitor and Maintain LAPB and X.25



X.25 and LAPB Configuration Examples

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Cisco IOS 12.0 Wide Area Networking Solutions

From: Cisco IOS 12.0 Wide Area Networking Solutions
Author: Technologies Riva; Systems Cisco
Publisher: Cisco Press (53)
More Information

7. Configuring X.25 and LAPB

This chapter describes how to configure connections through Link Access Procedure, Balanced (LAPB) connections, and X.25 networks. LAPB procedures are presented first for those users who only want to configure a simple, reliable serial encapsulation method. This chapter also describes how to configure Defense Data Network (DDN) X.25 and the Blacker Front End (BFE) and how to create X.29 access lists.

For a complete description of the commands mentioned in this chapter, see Chapter 8, “X.25 and LAPB Commands.” To locate documentation of other commands that appear in this chapter, you can search online at

LAPB Configuration Task List

You can use only LAPB as a serial encapsulation method if you have a private serial line. You must use one of the X.25 packet-level encapsulations when attaching to an X.25 network.

The LAPB standards distinguish between two types of hosts: data terminal equipment (DTE), and data circuit-terminating equipment (DCE). At Level 2, or the data link layer in the OSI model, LAPB allows for orderly and reliable exchange of data between a DTE and a DCE. A router using LAPB encapsulation can act as a DTE or DCE device at the protocol level, which is distinct from the hardware DTE or DCE identity.

Using LAPB under noisy conditions can result in greater throughput than High-Level Data Link Control (HDLC) encapsulation. When LAPB detects a missing frame, the router retransmits the frame instead of waiting for the higher layers to recover the lost information. This behavior is good only if the host timers are relatively slow. In the case of quickly expiring host timers, however, you will discover that LAPB is spending much of its time transmitting host retransmissions. If the line is not noisy, the lower overhead of HDLC encapsulation is more efficient than LAPB. When you are using long-delay satellite links, for example, the lock-step behavior of LAPB makes HDLC encapsulation the better choice. To configure LAPB, complete the tasks in the following sections. The tasks in the first section are required; the remaining are optional.

  • Configure a LAPB Datagram Transport

  • Modify LAPB Protocol Parameters

  • Configure Priority and Custom Queuing for LAPB

  • Configure Transparent Bridging over Multiprotocol LAPB

To monitor and maintain LAPB, refer to the section “Monitor and Maintain LAPB and X.25” later in this chapter. Examples of LAPB configurations are given at the end of this chapter.


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