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.
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.