cisco cdp

Posted on 2002-05-01
Last Modified: 2010-05-18
Hi guys

I have a cisco 4003 and cisco 7200 router seperated with a HP switch.

I often use CDP which I believe multicasts ( or is that E/IGRP?

Will the mulicasts (I think) pass thru the hp switch? Because I cannot see the other devices via CDP. Or they have to be DIRECTLY connected?

Question by:dbrannigan
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Expert Comment

ID: 6983657
That is EIGRP.  CDP is considered "media and protocol independent" so I don't think it comes out as an IP multicast.  The only info I could find said that it multicasts to the "common destination address 01-00-0C-CC-CC".  

If your switch is blocking multicasts you have a problem.  It is not the function of a switch to block multicasts or broadcasts.  That is a router's job.  A multicast should be treated like a broadcast and sent out all ports.  Is it a manageable switch?  Does it have multicast options (like an option to block them for some crazy reason)?

Author Comment

ID: 6983723
its a HP procurve switch. I do apologise there are two of this switches in between. You are correct it does not use multicasts. Would you expect the CDP broadcasts to passthru the switches so the cisco 7200 sees the catalyst 4003? or would the 7200 ethernet side have to be plugged into the switch?


Expert Comment

ID: 6983779
Well, I didn't mean to give the impression that CDP does not use multicasts - it just can't necessarily be considered an IP multicast.  It still multicasts to a hardware address.  Multicasts have typically the same network boundaries as do broadcasts - so multicasts should flow freely through the broadcast domain.  I would use a sniffer to view the packets as they leave each port if you can monitor in that way on your switch.  Othewise, if you can temporarily connect the two via a hub to see if it works that way, and then if it doesn't through the switch, you know where your problem is.
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Expert Comment

ID: 6984757
I can't say for the HP switch...but with Cisco switches...unless you activate "cgp enable" you wind up with multicasts hitting every port in the switch.  You might want to check to see if the HP switch has an equivilant of the cgp enable.  Heads up.

Accepted Solution

dobbo earned 100 total points
ID: 6988511
CDP will only show you directly connected devices.

CDP is enabled by default in all cisco devices.

CDP can discover other devices as long as the connection does not go through any non cisco (ie non CDP) device.

Author Comment

ID: 6993404
Thanks for all your help guys.. appreicate it.

Expert Comment

ID: 6993750
Just an FYI - dobbo's answer is not fully correct.  CDP sends messages at the data-link layer.  Therefore, regardless if a switch or hub is Cisco equipment or not, the messages will travel through it.  However, if your switch for some reason does not know how to handle multicasts, than you have a problem.  This would not be a CDP problem, but a problem with the configuration or abilities of your switch and would have problems other than just with CDP.


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