What would cause the output to show only the single neighbor?

A network engineer notices inconsistent Cisco Discovery Protocol neighbors according to the diagram that is provided. The engineer notices only a single neighbor that uses Cisco Discovery Protocol, but it has several routing neighbor relationships.

What would cause the output to show only the single neighbor?
A . The routers are connected via a Layer 2 switch.
B . IP routing is disabled on neighboring devices.
C . Cisco Express Forwarding is enabled locally.
D . Cisco Discovery Protocol advertisements are inconsistent between the local and remote devices.

View Answer

Answer: A

Explanation:

If all of the routers are connected to each other using a layer 2 switch, then each router will only have the single switch port that it connects to as its neighbor. Even though multiple routing neighbors can be formed over a layer 2 network, only the physical port that it connects to will be seen as a CDP neighbor. CDP can be used to determine the physical topology, but not necessarily the logical topology.