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