CCNA Certification Test Prep Case Study:
The DTE/DCE Network Cable And Directly Connected Serial Interfaces
By Chris Bryant, CCIE #12933
Your Cisco CCNA certification training can't just consist of routing protocol study - you've got to know the different kinds of network cables and Cisco router configurations running in today's networks, and the simplest looking configurations can be rich in details! One such cable is the DTE/DCE cable, and one such detail-rich configuration is needed when we connect two Cisco routers directly using serial interfaces. This is also a typical configuration in CCNA and CCNP home labs as well, particuarly when setting up a frame relay switch.
In this example, we've connected Router1 and Router3, using interface Serial1 on both routers.
By default, Cisco routers are DTEs (Data Terminal Equipment). This poses a problem when we connect them directly, since one of the routers must serve as the DCE (Data Circuit-Terminating Equipment) in such a connection. The DCE must send a clock rate to the DTE, so we've got to configure that as well.
There will be a DTE/DCE cable connecting these routers, with the DTE end of the cable naturally connected to the DTE and the DCE end... well, you know where that goes. But what if the cable is already connected and you need to ensure that the DCE end is indeed connected to Router3? Current DTE/DCE cables have "DTE" and "DCE" physically stamped into the connector itself, but what if you can't see that?
In this case, the Cisco router command show controller is our friend! There's a lot of output from this command, but the information pertinent to our situation is right at the top:
R1#show controller serial 1
HD unit 1, idb = 0x1DBFEC, driver structure at 0x1E35D0
buffer size 1524 HD unit 1, V.35 DTE cable
If R1 is connectd to the DTE end of the cable, R3 must be connected to the DCE end.:
R3#show controller serial 1
HD unit 1, idb = 0x1C44E8, driver structure at 0x1CBAC8
buffer size 1524 HD unit 1, V.35 DCE cable
I mentioned earlier that the DCE must supply a clock rate to the DTE. If this step is left out, the physical connection can be opened but the logical connection, the line protocol, will fail.
R3#show int s1
Serial1 is up, line protocol is down
Once we have a clock rate configured on R3, the line protocol should come up. Let's configure a clock rate and find out!
R3(config)#int s1
R3(config-if)#clockrate 56000
1d06h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to up
For such a simple Cisco router configuration, there's a lot going on here! Part of your CCNA certification training is mastering these details, so knowing how to find out which end of the cable is connected to the DCE and knowing what the DCE needs to do to make this configuration valid couldn't hurt a bit!
To your success,
Chris Bryant
CCIE #12933
chris@thebryantadvantage.com
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