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.
Incorrect Answers:: CDP operates at layer 2. The physical layer is layer 1.
: CDP operates at layer 2. The session layer is layer 5.
: CDP operates at layer 2. The application layer is layer 7.
: CDP operates at layer 2. The network layer operates at layer 3.
Reference: “Configuring CDP”
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.1E/native/configuration/guide
/cdp.html