How to configure Router Serial Interfaces, DTE (Data Terminal Equipment) and DCE (Data Communications Equipment)

When connecting a serial cable to the serial interface of the router, clocking is provided by an external device, such as a CSU/DSU device. A CSU/DSU (Channel Service Unit/Data Service Unit) is a digital-interface device used to connect a router to a digital circuit. The router is the DTE (Data Terminal Equipment) and the external device is the DCE (Data Communications Equipment), where the DCE provides the clocking. However, in some cases we might connect two routers back-to-back using the routers’ serial interfaces (Example: Inside the router labs). Each router is a DTE by default. The cable decides which end to be DCE or DTE and it is usually marked on the cable. The picture below shows back to back cable.

Smart Serial to Smart Serial back to back cable

If is not marked, we can use the "show controller" command to determine the interface is DTE or DCE. Since clocking is required to enable the interface, one of the two routers should function as DCE and should provide clocking. This can be done by using the "clock rate" command, from the interface configuration mode.

Router(config)# interface s0
Router(config-if)# clock rate 64000

Some of the possible values for clock rate are 1200, 2400, 4800, 9600, 19200, 38400, 56000, 64000, 72000, 125000, 148000, 500000, 800000, 1000000, 1300000, 2000000, and 4000000.

These values are in bites per second. You can find the possible values by using help.

Router(config-if)# clock rate ?

All interfaces have a bandwidth value assigned to them. This is used by certain routing protocols, such as IGRP, OSPF, and EIGRP, when making routing decisions. For LAN-based interfaces, the speed of the interface becomes the bandwidth value (In kilobits per second (Kbps)). However, on synchronous serial interfaces, the bandwidth defaults to 1,554Kbps and this is not related with the clock rate set. To change the bandwidth value for an interface, use the bandwidth Interface Subconfiguration mode command:

Router(config)# interface s0
Router(config-if)# bandwidth 64