User guide

A-16 Cisco 2500 Series Access Server User Guide

LANs and WANs

Note The cable used on the access server port will conﬁgure the port as DTE or DCE. If

it is conﬁgured as DCE, enter the command clockrate value to enable the port for use as a

DCE port.

Some WAN technologies, such as Frame Relay, Switched Multimegabit Data Service

(SMDS), or X.25, reuse the DTE and DCE terminology at Layer 2. The DTE/DCE mode

at Layer 1 is independent of the DTE/DCE mode at Layer 2. For example, It is possible for

a single device to be a V.35 DTE and a Frame Relay DCE concurrently.

If the access server is conﬁgured as a DTE (default) and is correctly receiving clocking and

Carrier Detect (CD) signals from the CSU, the command show interface will display the

following information:

Router# sh interface

Serial0 is up, line protocol is down

Note In this example, the Serial 0 interface is up, but the line protocol is reported to be

down. For the line protocol to be up, the WAN data link (Layer 2) must also be operational.

WAN Data Link Layer

The WAN data link layer (Layer 2), deﬁnes how data is formatted, or framed, for

transmission to remote sites. This formatting is referred to as encapsulation. Common

WAN encapsulations include High-Level Data Link Control (HDLC), Point-to-Point

protocol (PPP), Frame Relay, and SMDS.

WAN Layer 2 technologies can be grouped into two categories: point-to-point HDLC and

PPP) and multipoint (Frame Relay, SMDS, X.25, and Asynchronous Transfer Mode

(ATM)). Each category has its own speciﬁc design considerations. Multipoint technologies

can also be used in point-to-point topologies.