Cisco 2501 - Router - EN Bedienungsanleitung PDF herunterladen (Seite 44)

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Chapter 4

Introduction to the Cisco IOS and SDM

Serial Interface Commands

Wait! Before you just jump in and configure a serial interface, you need some key infor-

mation—like knowing that the interface will usually be attached to a channel service unit/

data service unit (CSU/DSU) type of device that provides clocking for the line to the router,

as I’ve shown in Figure 4.5.

Here you can see that the serial interface is used to connect to a DCE network via a CSU/DSU

that provides the clocking to the router interface. But if you have a back-to-back configuration

(for example, one that’s used in a lab environment like the one I’ve shown you in Figure 4.6), one

end—the data communication equipment (DCE) end of the cable—must provide clocking!

By default, Cisco routers are all data terminal equipment (DTE) devices, which means that

you must configure an interface to provide clocking if you need it to act like a DCE device.

Again, you would not provide clocking on a production T1 connection, for example, because

you would have a CSU/DSU connected to your serial interface, as Figure 4.5 shows.

F I G U R E 4 . 5 A typical WAN connection

F I G U R E 4 . 6 Providing clocking on a nonproduction network

DTE

Clocking is typically provided by DCE network to routers.

In nonproduction environments, a DCE network is not always present.

DCE

DTE

DCE

DCE side determined by cable.

Add clocking to DCE side only.

Set clock rate if needed.

Todd#config t

Todd(config)#interface serial 0

Todd(config-if)#clock rate 64000

show controllers will show the cable connection type.

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Cisco 2501 - Router - EN Bedienungsanleitung Seite 44