Two Cisco routers attach to a Catalyst. On one router you type show cdp neighbor. You expect to see the other router listed because cdp announces on a multicast address 01-00-0c-cc-cc-cc, which is flooded by bridges. But you see only the Catalyst as a neighbor. Suspecting that the other router isn’t generating cdp announcements, you enter show cdp neighbor on the Catalyst. You see both routers listed verifying that both routers are generating announcements. Why didn’t you see the second router from the first router? (Hint: neither router can see the other.)
CDP announcements use 01-00-0C-CC-CC-CC as the destination address. The Catalyst receives these announcements and terminates them in the Supervisor module. It does not forward them to other interfaces. Careful examination of the router’s cdp neighbor display will show only one neighbor, the Catalyst since it generates cdp announcements.