Cisco IOS Cookbook, 2nd Edition

Using an Internal 56 Kbps CSU/DSU

Problem

You want to configure an internal 56 Kbps CSU/DSU.

Solution

The configuration for an internal 56 Kbps CSU/DSU is similar to that of an internal T1 CSU/DSU:

Router2#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#interface Serial0/1
Router2(config-if)#ip address 192.168.99.25 255.255.255.252
Router2(config-if)#no shutdown 
Router2(config-if)#service-module 56k clock rate 9.6
Router2(config-if)#exit
Router2(config)#end
Router2#

Discussion

There are several options available for configuring internal 56 Kbps CSU/DSU cards in a router, and they all use the service-module command, similar to the T1 module. The above example shows how to set the CSU/DSU to a line speed of 9,600 bps using the clock rate option. This option takes as an argument the line speed in Kilobits per second, with valid options being 2.4, 4.8, 9.6, 19.2, 38.4, 56, or 64. The default line speed is 56 Kbps. You can also configure the module to automatically adapt to whatever the line speed might be using this auto keyword:

Router2(config-if)#service-module 56k clock rate auto

This is particularly useful if the line speed frequently changes for some reason.

Note, however, that this clock rate option does not imply that the internal CSU/DSU is the clock source for the circuit. By default, the CSU/DSU will assume that the clock signal comes from the network. If you want your router to supply the clock signal instead, you must configure it ...