CDP neighbours

[rakeshvelagala]

Hi

Just wondering if a setup is as below

cisco switch ted ( fas 1/2)-----------(port 1)linksys switch router(Port 2)---------(int fas 3/1)cisco switch red

when I do a cdp neighbour on one of the switches, will i see other switch as neighbour? If say i did it on switch ted, will it show a neighbour directly connected to fas 3/1 ??

Please advise

[John Blakley]

It depends on your Linksys router. If it passes the cdp packet, then you'll see the cisco switch on the other end. If it doesn't pass the packet, then you won't see neighbors at all. I'm going to assume that it will pass it fine, so you'll see the Cisco switch on the other side, but you won't see the Linksys because it probably doesn't support cdp but will pass the information along.

HTH,
John

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[Reza Sharifi]

Never tried that before, but I don't think you will see them as neigbors since there is Linksys device in the middle.

HTH

[rakeshvelagala]

Hi 

I came across this setup, where, when we check the cdp neighbour on one of the switch it showed cisco switch as connected to it!!! and since we have not given any ip in the switch, it showed linksys IP as the ip of the switch!

Regards 

[Peter Paluch · Accepted solution]

Hi,

CDP packets should pass transparently through every non-Cisco switch because they are sent to a specific multicast MAC address. As non-Cisco switches do not listen on this multicast MAC address, they flood the frame just like any other frame towards an unknown destination.

CDP messages will not pass through routers (or better said, through routed ports) because they are not carried in IP packets and therefore they can not be routed from one network into another. The propagation of CDP messages is always limited to at most a single broadcast domain.

Also, if the Linksys device in the middle does not understand CDP, you will not see it in the show cdp neighbors output. Instead, you should see the TED and RED switches in the CDP listings directly.

Best regards,
Peter

[Julio Carvajal]

Great explanation from Peter as always,

Is there a way u can run the command (Hope u actually have that network setup hehe).

I am interested to see the result. Never tough of that (I have seen Q-in-Q to make this happen) but never natively across another Switch that does not understand CDP.

Regards,