Cisco IOS Cookbook, 2nd Edition

Using an Internal T1 CSU/DSU

Problem

You want to configure an internal CSU/DSU for a WAN connection.

Solution

Cisco has a variety of different types of internal CSU/DSU devices that you can install in a router. In the following example, we have configured the internal CSU to support a fractional T1 circuit:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#interface Serial0/1
Router1(config-if)#ip address 192.168.99.9 255.255.255.252
Router1(config-if)#no shutdown 
Router1(config-if)#service-module t1 timeslots 1-12
Router1(config-if)#exit
Router1(config)#end
Router1#

Discussion

All of the work here is done with the service-module command:

Router1(config-if)#service-module t1 timeslots 1-12

This example tells the internal CSU/DSU to use the first 12 time slots of the T1 circuit. The dash character, “-”, tells the router to use a range of values. If you have a more complicated list of time slots, you can configure the list by using a list of ranges separated by commas. For example:

Router1(config-if)#service-module t1 timeslots 1-3,5-19,21

Or, you could even configure all of the odd-numbered time slots, as follows:

Router1(config-if)#service-module t1 timeslots 1,3,5,7,9,11,13,15,17,19,21,23

If you want to use all of the time slots, you can just use the keyword all.:

Router1(config-if)#service-module t1 timeslots all

By default, the CSU will assume that all of these time slots are 64 Kbps DS0 channels. But if the circuit actually uses 56 ...