CSU/DSU - Connectivity in Detail

Point-to-Point Serial Links

In the network at your job, most likely the serial interfaces on your

Cisco routers are not connected to each other directly. They connect

to a CSU/DSU, which supplies a clock rate to the router, allowing the

line protocol to stay up.

In the world of Cisco exams, and in your practice lab, there are

generally routers that have directly connected serial interfaces. These

routers are connected to each other by a DCE/DTE cable; the DCE end

of the cable will connect to the router that is acting as the CSU.

What’s The Line Protocol?

You’ll see a lot of discussion in CCNA and CCNP texts, but rarely does

anyone actually say what the line protocol is. The Cisco IOS Command

Reference defines the line protocol as "indicating whether the software

processes that handle the line protocol consider the line usable (that

is, keepalives are successful) or whether it has been taken down by an

administrator."

Translation: When the line protocol is down, there’s a problem with

the keepalives or the encapsulation type.

To tell the DTE end from the DCE end before connecting it, look for a

small label wrapped around one or both of the cable ends. That label

will indicate whether that is the DCE or DTE end. If there is no label,

the connector itself may have DTE or DCE imprinted on it.

After connecting the cable to the respective routers, use show

controller to ensure the router sees the cable as a DCE or DTE.

R1#show controller serial 1

HD unit 1, idb = 0x107114, driver structure at 0x10C590

buffer size 1524 HD unit 1, V.35 DTE cable

R3#show controller serial 1

HD unit 1, idb = 0xC7D1C, driver structure at 0xCCAA0

buffer size 1524 HD unit 1, V.35 DCE cable

The routers will not be able to communicate at this point, however.

Remember that when a serial interface connects to a CSU/DSU, the

interface receives clocking from that device. There is no CSU/DSU

involved when two serial interfaces are directly connected; therefore,

one of the devices must supply a clock rate to the other. The DCE

interface must supply the clock rate to the DTE.

R3(config)#int serial 1

R3(config-if)#clockrate 56000

<>

%LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to up

R3#ping 172.12.13.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.12.13.1, timeout is 2 seconds:

!!!!!

In The REAL World…

For exam purposes, you need to memorize the fact that the DCE is the

interface that needs to have the clock rate configured. When you’re at

your practice rack, you’ll find out that you can’t put the clockrate on

the DTE, because the router won’t let you!