How does CDP determine which IP to advertise?

michaelh · ‎12-03-2009

When looking at output from a "show cdp neighbor detail" command, what determines what IP address(es) appear under the Entry address(es) section? Is it something that can be configured, and can you have it advertise all addresses associated with that neighbor? The same goes for the information under Management address(es). I assume because of the "(es)" it is possible to have a situation with multiple entries, yet I have never seen that occur.

Joe Clarke · ‎12-03-2009

CDP will use the local interface's IP address by default. Newer versions of IOS offer the "cdp source-interface" command to specify what interface's IP to use for advertisements.

michaelh · ‎12-03-2009

Ok, to be a bit more specific, what happens in the case of a trunked interface, with IPs assigned to multiple VLANs on that trunk? Since posting I did discover that cdp source-interface command, which is helpful. I guess I'm still interersted in how IOS decides in situations where its not explicitly configured.

Joe Clarke · ‎12-03-2009

The IP of the native VLAN is used.