RouterGod Celebrity Lecture Series
Juliette Lewis On Troubleshooting Frame Relay
When you live in Los Angeles like the staff here at RouterGod Online Magazine does, you never know when you're going to run into a celebrity. RouterGod field reporter Alfred Saulo spotted Juliette roller blading on the Santa Monica pier wearing only a pink string bikini. Always quick to befriend a naked celebrity with a hot body, Alfred struck up a conversation and discovered that not only did Juliette understand Frame Relay, but she has two constipated pet Chihuahuas named FECN and BECN! Only in Los Angeles! Juliette has appeared in over 18 movies including Kalifornia, Natural Born Killers and What's Eating Gilbert Grape. She agreed to present a lecture on the subject of troubleshooting frame relay. And now, Juliette Lewis:
Hey you guys, how are you all doing? Anyway, why am I here? Oh yeah...I'm gonna show ya how to troubleshoot frame relay on Cisco routers. Yeah...right.
OK, I'm gonna show you some totally awesome show commands and some debugging commands too.
First off, how do you know your frame relay link is not working? The users say so? Don't go by what they say, try to ping your the router interface on the other side of the link and see for yourself. Half the time users say the network is down it really means they haven't turned on their monitor.
Whenever I'm having trouble on a frame PVC the first thing I like to do is a show interface serial1 or whatever serial interface is connecting the router to the DSU/CSU. You want to start troubleshooting this connection, then work you way outward. You want to see "up and up" and the LMI type 'cause it has to match the type the Telco is using. Also don't forget you wanna see "frame relay" as the encapsulation type, not HDLC or something like that. This command tells you about the physical layer.
The next thing I do is use the show frame-relay lmi command. LMI's are keep alives that are sent every 10 seconds by the phone company's frame switch. They use DLCI 1023 and when you issue this command you will see the LMI count incrementing to indicate that the LMI's are being received. Basically you enter the command a couple of times and hope the numbers get bigger.
Another thing that gets out of hand is frame relay mapping. Mapping means relating a layer 3 address to a layer 2 address. This means mapping an IP address to a DLCI number. You can let inverse ARP do the mapping or you can do it manually using the frame-relay interface-dlci command but how would you actually check to make sure it is correct? Use the show frame-relay map command to verify this mapping.
The fourth and final show command is show frame-relay pvc. This tells you if the PVC is active, what DLCI it's using and it will report on the number of BECNs and FECNs it has received.
Here is a summary of the 4 show commands we use to troubleshoot frame relay:
The last words of each command can be arranged to spell L.I.M.P. That's Lmi, Interface, Map and Pvc. Remember this, you might be tested on it! Picture the doctor asking to show him why you limp.
Now we will cover the various debugging commands that actually let you little buggers see what's being sent and received by the router in real time!
Debugging a router can cause it to get so busy doing it's little debugging show that it will stop routing so be careful debugging production routers. Tell your customer that their router may require a re-boot, that way you're covered in case you crash their core 7513. Or blame it on the phone company, that works too. The point is, you might trash their network, so before doing any work, think up a plausible excuse. If the guy who originally configured the router is not present, start blaming him even before you look at the config. I'll leave it to you, but experienced technicians can just look at a router from across the room and start making slanderous accusations.
Ok, just like there were 4 show commands we used, there are 4 debug commands we use to troubleshoot frame relay. The first is debug frame-relay events. This will show what happens every time something happens on your frame relay link. Broad in scope, it consumes many CPU cycles. A guaranteed show stopper.
The next command is debug frame-relay lmi. This lets you marvel at the wonder that is the LMI protocol. You wanna see LMI's with increasing myseq's and yourseq's.
Here's a good one! debug frame-relay packet. This will display every frame relay packet that crosses your frame relay interface. Better capture this because it will scroll by so fast your head will spin! This will probably crash a medium sized network, running this command on a large network may get you arrested.
The last debug command is debug serial interface. This shows you sequencing information and checks the line and DTE status. So here are the 4 debug commands:
The last words of each command start with E.L.P.I. You can remember them by the phrase "Emerson, Lake and Palmer In concert". Picture Keith Emerson playing the piano and hundreds of bugs crawling up his legs. When I ask you what 4 debugging commands are used in frame relay, what are you going to say?
Don't forget that all this buggery slows down the router, when you are done, enter no debug all.
Well that's it for this weeks lecture, you need to study all this stuff not only to pass the Support test, but knowing all this stuff will increase your pay and make you more appealing to the opposite sex.
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