You enabled CDP on two Cisco Routers which are connected to each other. The Line and
Protocol status for the interfaces on both routers show as UP but the routers do not see
each other a CDP neighbors. Which layer of the OSI model does the problem most likely
exist?

A.
Physical

B.
Session

C.
Application

D.
Data-Link

E.
Network

Explanation:

CDP is a protocol that runs over Layer 2 (the data link layer) on all Cisco routers, bridges, access
servers, and switches. CDP allows network management applications to discover Cisco devices
that are neighbors of already known devices, in particular, neighbors running lower-layer,
transparent protocols. With CDP, network management applications can learn the device type and

the SNMP agent address of neighboring devices. This feature enables applications to send SNMP
queries to neighboring devices. In this case, the line protocol is up which means that the physical
layer is operational (layer 1) but the data link layer is not.
Reference: “Configuring CDP”
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.1E/native/configuration/guide/c
dp.html