Cisco CCNA / CCNP / CCENT Home Lab Tutorial:

When, Where, And Why To Use DTE/DCE Cables

By Chris Bryant, CCIE #12933

More Cisco CCNA, CCENT, and CCNP candidates than ever before are putting together their own home labs for their certification exam study, and that's a great trend - there's nothing like learning on the real thing!

Part of putting a home lab together is getting the right cables and understanding their usage. In this new Cisco home lab series, we'll take a look at the different cable types and how each fits into your home lab. The first cable type we'll look at is the multi-purpose DTE/DCE cable.

When I say "multi-purpose", I mean that while the cable will always perform the same task, it can be used in several different points in your home lab network. If you're going to have a frame relay switch - and you should get one if at all possible, since having your own frame relay cloud is a tremendous boost to your home lab studies and your exam score - you're going to need a DTE/DCE cable. Actually, more than one!

In network diagrams, a frame relay cloud is generally illustrated by just that - a cloud:

In your Cisco home lab, an entire frame relay cloud can be simulated with one single router. Since we have three other routers, we'd need three DTE/DCE cables to physically set up the network.

Each of those links is a DTE/DCE cable, with the router serving as the frame relay cloud configured to act as the DCE on each link.

You can also use a DTE/DCE cable to directly connect two Cisco router serial interfaces and configure HDLC (the default) or PPP encapsulation over that point-to-point link. In our home lab, the connection symbolized below would physically be a DTE/DCE cable.

Most of today's DTE/DCE cables have "DTE" clearly stamped on one end of the cable - actually, "DTE" is probably embedded into the connector itself. Naturally, the other end will have "DCE" clearly indicated. It's the DCE end that will connect to your frame relay switch. If you're going to use a direct connection to run PPP or HDLC, it almost doesn't matter which end of the cable is connected to a given router.

Keyword: "almost". You must use the clockrate command on the DCE end of the connection in order to bring the line protocol up.

If you're not sure which end of your DTE/DCE cable is connected to a given serial interface, just use the show controller serial command to get that information. Most of the output of that command isn't comprehensible, but what we need is right on top:

R3#show controller serial 1
HD unit 1, idb = 0x11B4DC, driver structure at 0x121868
buffer size 1524 HD unit 1, V.35 DCE cable

Naturally, if it's the DTE end, you'll see "DTE" there. And if you don't have anything connected to that interface, you'll see "no cable".

Whether you have your own frame relay switch or not, you'll want to pick up some DTE/DCE cables for direct connections between your Cisco home lab router's serial interfaces. Just don't forget to put the clockrate command on the DCE end of the cable!

And if you're not familiar with a frame relay switch, just check out that link!

Five Minutes From Now, You Can Be Studying For CCNA Exam Success Just As Thousands Of Other CCNA Candidates Around The World Have - With Chris Bryant, CCIE #12933 With You Every Step Of The Way - With The Ultimate CCNA Exam Study Package!