Bandwidth Clock Rate and Serial Lines in the

As mentioned back in Chapter 4, "Fundamentals of WANs," you can build a WAN link in a lab without using a CSU/DSU. The lab network that I used to build the examples in this chapter used three "back-to-back" serial cables, essentially a DTE and DCE cable pair connected together.

To use a back-to-back WAN connection, one router must supply the clocking. Example 13-7 shows an example configuration for Seville, with a couple of important commands related to WAN links.

Example 13-7 Seville Router Configuration with clock rate Command

Example 13-7 Seville Router Configuration with clock rate Command

Clock Rate Bandwidth

The clock rate command sets the rate in bits per second on the router that has the DCE cable plugged into it. In this case, Seville was supplying clocking on both serial interfaces. If no cable has been plugged in, the IOS accepts the command. If a DTE cable has been plugged in, IOS rejects the command. If you do not know which router has the DCE cable in it, you can find out by using the show controllers command, as shown at the end of the example. In the example, you can see that the output identifies the type of serial cable.

Also notice the bandwidth 128 command on serial 1. The bandwidth command tells IOS the speed of the link, in kilobits per second, regardless of whether the router is supplying clocking. The bandwidth setting does not change anything that the router does at Layer 1; instead, this setting is used by IOS software for other purposes. For instance, IGRP and EIGRP both use bandwidth to calculate a metric for routing protocols; they use the bandwidth setting on the interfaces. bandwidth defaults to T1 speed on serial interfaces. There is no default for clock rate, even with a DCE cable plugged in—it must be configured.

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