Question: CDP over Frame Relay

Unanswered Question
Jun 6th, 2007

Hi all,

I have a 2500 configured as a Frame-Relay switch with two other routers configured on either side. Now, I understand that FR is of NBMA type. Can I assume correctly that CDP will not work in this case?

I tried configuring it, however, I am getting the following error: CDP-EV: Encapsulation on interface Serial0 failed

On the other hand, CDP works below layer 3, so it should not in principle rely on the FR.

Other clarifications:

The two routers on either side on the switch (i.e. the router configured as a FR switch) are able to ping each other and show each other as CDP neighbours.

Can somebody please clarify this for me?

Kind regards.

I have this problem too.
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tperrier Wed, 06/06/2007 - 23:47

CDP will transit through the Frame Relay PVC carried by the FR switch. By default it's disabled on FR interfaces, but you can enable it. Then the end point routers will see one another, but you won't see the intermediate frame switch(es).

network_enginee... Thu, 06/07/2007 - 00:45


Thanks for that response. Yes, I enabled it on the serial interfaces on the end point routers. These serial interfaces are using Frame Relay encapsulation.

That besides the point, I get what you are saying. But then CDP working at layer 2 should also be able to discover the switch isn't it? I also have two router ethernet interfaces connected over a L2 2900 XL switch and the routers show the switch as its CDP neighbour.


srue Thu, 06/07/2007 - 06:17

Your routers will only show the switch as their cdp neighbors, that is the expected behavior. Frame-relay switches however, pass CDP through them, so the routers on either end of a dlci should see each other as cdp neighbors. CDP is only disabled on frame-relay point to multipoint interfaces by default, it's enabled elsewhere by default (i think).

sundar.palaniappan Thu, 06/07/2007 - 15:20

You are correct CDP works below layer 3. However, the problem is frame relay switch only switches packets between PVCs and doesn't originates data packets itself to be sent across the PVCs and that includes the multicast CDP packets.




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