To pass the CCNA exam, you've got to master quite a few services and routing protocols that may be new to you. Between RIP, IGRP, EIGRP, OSPF, and switching, there are hundreds of details you've got to absorb! It's easy to spend all your time on those topics and not pay proper attention to "easier" technologies, and then all of a sudden on exam day you can't quite remember the details of those particular services.
One setup you've got to be more than familiar with is directly connecting serial interfaces on Cisco routers. This is also a valuable skill to have in your home lab, since it allows you to add segments to your network setup.
A Cisco serial interface is operating as a DTE by default. The problem is that when you take a cable and connect two routers directly by their serial interfaces (with a DTE/DCE cable, that is!), they're both waiting for the other to send them a clock rate. One of the interfaces must act as the DCE and that interface must send the clock rate.
If you can see the DTE/DCE cable, you can tell by looking which router has the DCE interface connected to it - the letters "DTE" or "DCE" will either be molded into the connector itself, or if it's an older cable there should be a little piece of tape on the cable that tells you what the interface type is. But what if you have no access to the cable, or there are other cables all around it and you can't see what type it is?
Run the command "show controller serial x", with x representing the interface number the cable's connected to. There will be quite a bit of output from this command, but the information you need 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
I left off the 16 or so rows of information that comes after this, but this is the information we need right now. If R1's got the DTE cable end, the other router should have 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
We know now that R3 needs to supply a clock rate to R1. There's a hint of a problem in just that little bit of command output - do you see what it is? Let's run show interface serial1 to get more information.
R3#show int s1
Serial1 is up, line protocol is down
The line protocol is down because there is no clockrate being supplied by R3. If there has been, we would have seen that in the output of show controllers serial 1.
This is simple enough to fix, though! We'll use the command clockrate 56000 on R3's serial1 interface, and the line protocol will soon come up.
R3(config)#int s1
R3(config-if)#clockrate 56000
1w2d: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to up
This is a simple concept, but there are a few details you must keep in mind! For a home lab configuration, you'll need a DTE/DCE cable to make this work. If you cannot see the cable connectors, run show controllers serial x to see if the router has the DTE or DCE end of the cable attached. On the interface with the DCE attached, use the clockrate command to bring the line protocol up. It's just that simple!
CCNA exam success depends on mastering many technologies that are new to you, and few exam topics have more details than ISDN. ISDN isn't just for your CCNA exam studies, though. While ISDN is dismissed by many, the fact is that there are many small and mid-size networks out there that use ISDN as their backup to frame relay. Some of these companies have spoke networks that use ISDN to connect to their hub as well, so it's a great idea to know ISDN configuration and troubleshooting for your real-world career as well as passing the CCNA. With that in mind, let's take a look at five common ISDN errors and how to avoid them.
With dialer map statements, remember that the phone number you put in the dialer map is the phone number of the remote router, not the local one. Look at it this way - if you want to call a friend on your cell, you don't pick up your cell and dial your own number!
Speaking of dialer map statements, don't forget the all-important broadcast option at the end of the command:
R1(config-if)#dialer map ip 172.12.21.1 name R2 broadcast 5555555
The router will accept that command without the "broadcast" option, but routing protocol updates and hellos would not be able to travel across the line. (This command is also needed in frame relay map statements to allow broadcasts and multicasts to be transmitted.)
PAP is PPP's clear-text authentication scheme, and clear text is a really bad idea. But if you do have to configure it, don't forget that PAP requires additional configuration -the ppp pap sent-username command.
R1(config-if)#ppp pap sent-username R1 password CISCO
Must set encapsulation to PPP before using PPP subcommands
R1(config-if)#
The error message we got while configuring the sent-username command is another important reminder - by default, a BRI line is running HDLC, not PPP. Since HDLC doesn't allow us to use either PAP or CHAP, we'll need to set the link to PPP with the encapsulation ppp command.
R1(config-if)#encapsulation ppp
R1(config-if)#ppp authentication pap
R1(config-if)#ppp pap sent-username R1 password CISCO
But before we configure any of this information, we should configure the ISDN switch-type. Why? Because without the switch-type configuration, it doesn't matter that we avoid the other four errors - the line will not come up. Configure the switch-type with the "isdn switch-type" command, and then verify it with "show isdn status".
R1(config)#isdn switch-type basic-ni
R1#show isdn status
Global ISDN Switchtype = basic-ni (output of this command cut here for clarity)
If you forget this part of the configuration, the output of show isdn status wastes no time in reminding you!
R1#show isdn status
**** No Global ISDN Switchtype currently defined ****
ISDN is an important part of your CCNA studies, and this knowledge still comes in handy in production networks as well. Keep studying, notice the details, run those debugs, and you'll be a CCNA before you know it!
Chris Bryant has sinced written about articles on various topics from CISCO CCNA, Personal Desktop and Cisco CCNP. Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage , home of free and CCNP tutorials! Pass the. Chris Bryant's top article generates over 27100 views. to your Favourites.