CDP doesn't see neighboors through switch

[fsoffia]

hallo we have some cisco2600 routers directly connected to a Catalyst WS-C2950-24 switch IOS C2950-I6Q4L2-M 12.1(9)EA1.

The switch has default config.

The routers and the switch have CDP running.

On each router CDP see only the switch.

If we disable CDP on the switch on each router CDN does not see any neighboor !

The only way to see the other neighboors we sobstitute the switch with a hub.

Does CDP see only neighboors on the same collision domain ?

francesco

[pkhatri · Accepted solution]

Hi,

What you are seeing is correct operation. CDP works at layer2. When you place a layer 2 device (like a switch) between routers , CDP running on the routers will only see the switch as a neighbor.

When you insert a hub (which works at layer 1) in the middle, the layer 2 segment extends between the routers and therefore they can see each other as neighbors.

It is not a matter of collision domains but a matter of whether the intervening device is a layer 2 or a layer 1 device.

Hope that helps - pls rate the post if it does.

Paresh

[ankurbhasin]

Hi Francesco,

CDP only works for directly connected neighbor on layer 2. Now when you run a "sh cdp nei" on a router the directly neighbor is switch and it will not pass or multicast the cdp information further.

When you remove the switch and place the hub and run the sh cdp neighbor from the router all the routers are directly connected to your router as hub will broadcast the multicast request of CDP.

HTH, if yes please rate the post.

Ankur

[Richard Burts]

Making the point that CDP works at layer 2 is only part of explaining this behavior. It works if you put a hub in the middle and it would work if you put a non-Cisco switch in the middle.

What is happening is that the router sends CDP using the specific MAC address used for CDP. The Cisco switch sees that address as interesting traffic addressed to itself and processes it and does not forward it. If the switch were non-Cisco it would see a multicast destination and forward the frame along.

So this is normal behavior when the switch in the middle is a Cisco switch.

HTH

Rick

[aceqbaceq]

I know this is old topic. But  i 've been searching the answer for quite a long time!!!!

I did expreriment.

I got cisco router A  ------- non cisco switch --- cisco router B

and in this case  router A sees router B as a neighbor.

Ok.  i thought  it;s because cdp uses multicast.

So i replace  non cisco switch witch cisco switch. so the figure is

cisco router A  ------- cisco switch --- cisco router B

and in this case router A can't see router B as a neghbor.

Why? i couldnt get idea. if we have ,multicast it go all over L2 broadcast domain.

Only here  i got an answer that all cisco devices (including switches) doesn't multicast cdp frames.

From this moment all get together. 

Thank you very much.

Unfortunately Odom doesn't  enlight this very important moment .

[Rolf Fischer]

This behavior is typical for proprietary layer-2 control protocols as many of them use multicast destination MAC addresses (for instance UDLD, VTP, PAgP, SSTP BPDUs, etc.).

Only if the receiving device is aware of such a protocol, it will keep the communication link-local and act as a peer.

In contrast, a (3rd-party) device on which the protocol is not implemented won't consider itself as the recipient of the frame and treats it as normal multicast. That means in the case of a switch to replicate the frame over all ports in that VLAN, except for the port on which the original frame was received.

Especially in mixed environments this is very important to know!

[mheusinger]

Hello,

This is how CDP works. In case you have a Cisco device as a neighbor (L2 neighbor) it will show up, no matter if it is a switch or router. So your routers will discover the switch and vice versa.

Turning off CDP on a Cisco switch does not mean it will broadcast received CDP messages to all ports. It will silently discard them. This is the intended behaviour. CDP uses a specific MAC all Cisco devices are aware of and will not forward those messages.

The only exception I know off is EoMPLS, where a SP offers Ethernet connections to customers. There exists a configuration option in some platforms afaik, also to forward the CDP messages so that the SP device will not be visible to the customer.

Hope this helps! Please rate all posts.

Regards, Martin

[varun_gautam]

to the best of my experiences and

as goin with the layer approach

cdp on l2(switch) would see a switch and router will be a circuit breaker .