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Strange CDP neighbor output

I have a 7206 and a 2621 connected by point to point over frame-relay. I couldn't get to the 2621 with IP so I decided to check layer 2 connectivity and did a "sh cdp neigh detail" I was surpised to see the 2621 show up as a switch in the capability list, instead of a router. I'm running C2600 Software (C2600-IK9S-M), Version 12.2(8)T8. I checked a couple of other routers in a lab and found that when I turn off IP routing and then attempt to see them via CDP. The "R" capability code disappears and is blank. I can't seem to find anything that would explain this behavior. Why would IP routing being on or off have anything to do with CDP and why would a router show up as a switch?


Re: Strange CDP neighbor output

When you turn off ip routing, you are disabling all layer 3 functionality (all routing capabilities are turned off). In my opinion, it makes sense that the cdp info would remove the "R" capability code since at this point the router can not route packets.

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Re: Strange CDP neighbor output

In the test environment, this action of removing the "R" seemed to make sense, but this is not consistent along all 12.x versions of the code. In most versions of IOS, the router is still seen as a router.

Also, on the production 2621s, CDP is seeing the device as a "S". So in putting the two together we were thinking that perhaps IP Routing wasn't turned on in the remote production 2621, thereby causing CDP to see it as a switch. But given what we saw in the test lab, you would expect the capability to be blank rather than "S" if that were the case. So, now we are trying to determine how CDP at layer 2 can decide if a device is a switch, hub, router etc...

Re: Strange CDP neighbor output


This is normal behaviour. WHen routing is on, the neighbor is displayed as R.

When you turn of routing (no ip routing command), and clear the cdp table, you will see the neighbor entry with a blank, its neither a router nor bridge.

When you turn on bridging (transparent bridge using the command - bridge X protocol ieee), the neighbor will turn up as T (transparent bridge)

Hope that helps!@

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