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