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Cisco - Serial Interfaces

Hola!

to connect 2 Cisco routers via their serial interfaces, it can use a null serial cable connecting a DCE to a DTE.


By default, Cisco routers are DTEs (Data Terminal Equipment). This can be a problem when connecting 2 routers directly, because one of the two has to act as DCE (Data Circuit-Terminating Equipment) in this connection.

The DCE must send the clock rate to the DTE, so you have to configure it. Cables DCE / DTE are physically stamped on the connector itself to identify the corresponding letters, but what happens if we can not see? We can use the command show:

R1 # show controller serial 1 HD
unit 1, idb =..., driver structure ...
buffer size 1524 HD unit 1, V.35 DTE cable

R2 # show controller serial 1 HD
unit 1, idb = ..., driver structure ...
buffer size 1524 HD unit 1, V.35 DCE cable

Once we know what the team must act as DCE clockrate configure it to have it sent to DTE:

R2 (config) # interface serial 1
R2 (config-if) # ip address ...
R2 (config-if) # clockrate
56000 R2 (config-if) # no shutdown
R2 (config-if) # exit
R2 (config) # exit

And set the DTE:

R1 (config) # interface serial 1
R1 (config-if) # ip address ...
R1 (config-if) # no shutdown
R1 (config-if) # exit
R (config) # exit

That's it. Greetings!

Source: http://www.thebryantadvantage.com/CCNACertificationExamTutorialDirectlyConnectedSerialInterfaces.htm