CDP=>Receiving CDP packets from switches not directly connected.

Unanswered Question
Jun 28th, 2010

I have this weird problem where a specific switch port which happens to be part of a 2 switch 3750 Switch Stack is seeing multiple CDP packets from 3 extra switch port interfaces that are not directly connected. Noteworthy is that the far end devices have the correct CDP entries and I physically confirmed at least two of those connections that lead to the switch "upstream to the culprit switch". Tricky part is that its production so room for maneuvering is limited. At some point I disabled all Ports save for the real uplink and the problem momentarily disappeared. Re-enable the interfaces problem resurfaces. Is there an explanation, technique to eliminate the culprit with minimal disruption?

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Overall Rating: 5 (3 ratings)
Richard Burts Mon, 06/28/2010 - 06:22


I do not have a clear understanding of the topology in which this problem is happening. Could you clarify on the switch that is receiving CDP from not directly connected switches, what is the connection path? What device is this switch connected to on the receiving port? What is that device connected to that leads to the source of the CDP?

Is is possible that the device that connects the switches is a non Cisco switch (or hub)? In that case receiving CDP from not directly connected switches is expected behavior. CDP is sent as a layer 2 multicast address using a Cisco specific multicast MAC destination address. This address is understood by Cisco switches which then process the CDP packet. But non Cisco devices do not process this multicast and just flood it out their ports.

Is that perhaps the case here?



Rick Morris Thu, 07/26/2012 - 07:24


Are you saying that any device that is a non-Cisco (in my case Avaya) will reply back via multicast to the CDP request?  I am seeing devices that sit on the other side of the device that is a Cisco.  My topology is as follows:

Cisco 6509 ------ Avaya 5520 |-------- Cisco

                                          |-------- Cisco

                                          |-------- Cisco
And in my case when performing CDP on the 6509 I see the non-directly connected Cisco on the other side of Avaya 5520.



Richard Burts Thu, 07/26/2012 - 07:56


I do not want to be overly picky in answering your question but I do want to be precise. I am not saying that a non Cisco will "reply" to a CDP request. I am saying that a non Cisco (in your case Avaya) will pass the CDP through. So your 6509 would send a CDP frame out its interface. The Avaya would see it as a multicast (layer 2 multicast) and forward it. Your other Cisco will receive it and process it as CDP. Then the other Cisco gear will send CDP out its interface. It will get to the Avaya, which will forward it, and it is received and processed by your 6509. So the behavior that you are describing is very consistent with my explanation. Your 6509 and your other Cisco gear see each other by passing CDP traffic through the Avaya.



Nickolus Looper Wed, 10/07/2015 - 08:55

Great explanation - thanks.

Ran into an issue today where an access switch was learning the macs of connected access points, but the upstream L3 switch was not learning the macs, even though the L2 <-> L3 switch trunk was properly forwarding the correct VLANs.


Turned out that the APs were in a "special" state and only sending CDP frames.  So, those multicast frames were not forwarded upstream to the L3 switch, hence no AP macs in its mac address table.

Richard Burts Wed, 10/07/2015 - 10:46

I am glad that the explanation was helpful to you. The situation you faced seems quite unusual and understanding this aspect of where CDP is forwarded and where it is not forwarded would be crucial in figuring out the issue.





pwenger Tue, 01/03/2012 - 05:41

Hi Esmail

Have you ever fixed this issue? I got the same problem at customer site. He is using a Cat650X in VSS mode and to this VSS is a Cat3750 stack connected with a multichassis etherchannel. With a show cdp neigh g x/y on the VSS I see all the phones, accesspoints and ATA attached to the Cat3750 stack.




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