Using an Internal T1 CSU/DSU

Using an Internal T1 CSU DSU


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


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
Router1(config-if)#no shutdown 
Router1(config-if)#service-module t1 timeslots 1-12



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 Kbps channels, you can configure this by adding the speed keyword:

Router1(config-if)#service-module t1 timeslots 1-12 speed 56

Usually, in a production environment, the WAN carrier will provide the clock signal for a T1 circuit. But in some cases, particularly test and lab networks, you will need the internal CSU/DSU to act as the DCE device and supply the clock signal. You can configure the module to do this with the clock source internal keywords:

Router1(config-if)#service-module t1 clock source internal

By default, the router will use Binary 8 Zeroes Substitution (B8ZS) line coding. While B8ZS tends to be the most common way that T1 circuits are delivered, some service provides use Alternate Mark Inversion (AMI) instead. You can configure the internal CSU to use AMI line coding as follows:

Router1(config-if)#service-module t1 linecode ami

When you use AMI line coding, you have to either set the speed of each channel to 56 Kbps, as we did above, or use inverted data coding:

Router1(config-if)#service-module t1 data-coding inverted

And the other T1 option that you sometimes see is the use of Super Frame (SF) instead of the default Extended Super Frame (ESF). If your WAN vendor uses SF framing, you can configure this on your router with the following command:

Router1(config-if)#service-module t1 framing sf

Some network vendors require a special Facilities Data Link (FDL) configuration. There are three options. FDL can be disabled, it can be implemented according to the ANSI T1.403 standard, or it can use the AT&T standard. To enable the ANSI standard, you use the keyword ansi, as follows:

Router1(config-if)#service-module t1 fdl ansi

And you can turn on the AT&T option like this:

Router1(config-if)#service-module t1 fdl att

If you are using a WIC-1DSU-T1 module, FDL is disabled by default. However, on Cisco 2524 and 2525 routers, the built-in T1 CSU/DSU uses both ANSI and AT&T options simultaneously by default, and you can't disable FDL.

It is usually a good idea to enable the remote-alarm-enable option:

Router1(config-if)#service-module t1 remote-alarm-enable

This option allows the CSU to send remote alarms, also called yellow alarms, to the CSU on the other end of the circuit. It does this to let the other device know that it has encountered an alarm condition, such as a framing error or loss of signal on the circuit. You should only use this option with ESF framing because it conflicts with SF framing.

The following is a real-world configuration taken from a router installed in a remote area that only supported AMI line coding, SF framing, and ANSI standard FDL:

Router6(config)#interface Serial0/0
Router6(config-if)#service-module t1 framing sf
Router6(config-if)#service-module t1 linecode ami
Router6(config-if)#service-module t1 timeslots all speed 56
Router6(config-if)#service-module t1 fdl ansi


See Also

T1: A Survival Guide, by Matthew Gast (O'Reilly)

Router Configuration and File Management

Router Management

User Access and Privilege Levels


IP Routing





Frame Relay

Handling Queuing and Congestion

Tunnels and VPNs

Dial Backup

NTP and Time


Router Interfaces and Media

Simple Network Management Protocol





First Hop Redundancy Protocols

IP Multicast

IP Mobility




Appendix 1. External Software Packages

Appendix 2. IP Precedence, TOS, and DSCP Classifications


Cisco IOS Cookbook
Cisco IOS Cookbook (Cookbooks (OReilly))
ISBN: 0596527225
EAN: 2147483647
Year: 2004
Pages: 505 © 2008-2020.
If you may any questions please contact us: