| To pass the CCNA exam, you've got to master | | | | I left off the 16 or so rows of information |
| quite a few services and routing protocols | | | | that comes after this, but this is the |
| that may be new to you. Between RIP, IGRP, | | | | information we need right now. If R1's got |
| EIGRP, OSPF, and switching, there are | | | | the DTE cable end, the other router should |
| hundreds of details you've got to absorb! | | | | have the DCE end: |
| It's easy to spend all your time on those | | | | |
| topics and not pay proper attention to | | | | R3#show controller serial 1 |
| "easier" technologies, and then all of a | | | | |
| sudden on exam day you can't quite remember | | | | HD unit 1, idb = 0x1C44E8, driver structure |
| the details of those particular services. | | | | at 0x1CBAC8buffer size 1524 HD unit 1, V.35 |
| | | | DCE cable |
| One setup you've got to be more than familiar | | | | |
| with is directly connecting serial interfaces | | | | We know now that R3 needs to supply a clock |
| on Cisco routers. This is also a valuable | | | | rate to R1. There's a hint of a problem in |
| skill to have in your home lab, since it | | | | just that little bit of command output - do |
| allows you to add segments to your network | | | | you see what it is? Let's run show interface |
| setup. | | | | serial1 to get more information. |
| | | | |
| A Cisco serial interface is operating as a | | | | R3#show int s1 |
| DTE by default. The problem is that when you | | | | |
| take a cable and connect two routers directly | | | | Serial1 is up, line protocol is down |
| by their serial interfaces (with a DTE/DCE | | | | |
| cable, that is!), they're both waiting for | | | | The line protocol is down because there is no |
| the other to send them a clock rate. One of | | | | clockrate being supplied by R3. If there has |
| the interfaces must act as the DCE and that | | | | been, we would have seen that in the output |
| interface must send the clock rate. | | | | of show controllers serial 1. |
| | | | |
| If you can see the DTE/DCE cable, you can | | | | This is simple enough to fix, though! We'll |
| tell by looking which router has the DCE | | | | use the command clockrate 56000 on R3's |
| interface connected to it - the letters "DTE" | | | | serial1 interface, and the line protocol will |
| or "DCE" will either be molded into the | | | | soon come up. |
| connector itself, or if it's an older cable | | | | |
| there should be a little piece of tape on the | | | | R3(config)#int s1 |
| cable that tells you what the interface type | | | | |
| is. But what if you have no access to the | | | | R3(config-if)#clockrate 56000 |
| cable, or there are other cables all around | | | | |
| it and you can't see what type it is? | | | | 1w2d: %LINEPROTO-5-UPDOWN: Line protocol on |
| | | | Interface Serial1, changed state to up |
| Run the command "show controller serial x", | | | | |
| with x representing the interface number the | | | | This is a simple concept, but there are a few |
| cable's connected to. There will be quite a | | | | details you must keep in mind! For a home lab |
| bit of output from this command, but the | | | | configuration, you'll need a DTE/DCE cable to |
| information you need is right at the top: | | | | make this work. If you cannot see the cable |
| | | | connectors, run show controllers serial x to |
| R1#show controller serial 1 | | | | see if the router has the DTE or DCE end of |
| | | | the cable attached. On the interface with the |
| HD unit 1, idb = 0x1DBFEC, driver structure | | | | DCE attached, use the clockrate command to |
| at 0x1E35D0buffer size 1524 HD unit 1, V.35 | | | | bring the line protocol up. It's just that |
| DTE cable | | | | simple! |
| | | | |