CDP holdtimer command

Answered Question
Feb 20th, 2007

What does this feature really do?

Thanks

Reza

I have this problem too.
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Correct Answer by Richard Burts about 9 years 7 months ago

Reza

I understand the challenge of participating in something like NetPro when English is not your primary language. I feel that you have expressed clearly your question.

It has been a while since I tested this function and so I may not be entirely authoritative. But I believe that the difference is the reaction to situations where the CDP advertisements just stop and where the interface goes down. In the situation where the CDP packets just stop arriving the router should use the holdtime in CDP to maintain information about the neighbor. But when the interface goes down then the router knows that the neighbor is really gone and does not maintain the information for the holdtimer.

You can test this by bringing the serial interface back up, let CDP get established so that both routers see each other as neighbors, and then on R3 on the serial interface configure no cdp enable. This will stop CDP over the interface but will leave the interface up and active. In this case I believe that you will find that R1 maintains the information about R3 for the holdtimer.

HTH

Rick

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Richard Burts Tue, 02/20/2007 - 08:14

Reza

Normally CDP packets are sent on a periodic basis and as long as we continue to receive CDP packets on a regular basis we maintain the information about our neighbor. But what if we miss 1 CDP packet? Should we declare that the neighbor is down? (probably not - sometimes there is packet drop). So the holdtimer is used to determine how long we should maintain information on a neighbor when the periodic CDP packets are not being received.

HTH

Rick

rezaalikhani Tue, 02/20/2007 - 08:41

Thanks for your reply. Another question:

I have three routers: R1, R2 and R3. I have connected R1 to R3 by a back-to-back serial interface 1/0. When everything is Ok, I issue the "show CDP neighbors" command and I can see that the R3 is one of my entries. Then, I logins to the R3 and shuts the Serial 1/0 and immediately I come back to R1 and again "sh CDP neighbor" command, but at this time I cannot see the R3 from my routers entry. Is it normal? If so, why the R1 does not wait the entire holdtime, then flush its entry that relates to R3?

Sorry for my English writing.

Reza

Correct Answer
Richard Burts Tue, 02/20/2007 - 09:10

Reza

I understand the challenge of participating in something like NetPro when English is not your primary language. I feel that you have expressed clearly your question.

It has been a while since I tested this function and so I may not be entirely authoritative. But I believe that the difference is the reaction to situations where the CDP advertisements just stop and where the interface goes down. In the situation where the CDP packets just stop arriving the router should use the holdtime in CDP to maintain information about the neighbor. But when the interface goes down then the router knows that the neighbor is really gone and does not maintain the information for the holdtimer.

You can test this by bringing the serial interface back up, let CDP get established so that both routers see each other as neighbors, and then on R3 on the serial interface configure no cdp enable. This will stop CDP over the interface but will leave the interface up and active. In this case I believe that you will find that R1 maintains the information about R3 for the holdtimer.

HTH

Rick

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