CDP is a device
discovery protocol that runs over Layer 2 (the data-link layer) on all
Cisco-manufactured devices (routers, bridges, access servers, controllers, and
switches) and allows network management applications to discover Cisco devices
that are neighbors of already known devices. With CDP, network management
applications can learn the device type and the Simple Network Management
Protocol (SNMP) agent address of neighboring devices running lower-layer,
transparent protocols. This feature enables applications to send SNMP queries
to neighboring devices.
CDP runs on all media
that support Subnetwork Access Protocol (SNAP). Because CDP runs over the
data-link layer only, two systems that support different network-layer
protocols can learn about each other.
device sends periodic messages to a multicast address, advertising at least one
address at which it can receive SNMP messages. The advertisements also contain
time-to-live, or holdtime information, which is the length of time a receiving
device holds CDP information before discarding it. Each device also listens to
the messages sent by other devices to learn about neighboring devices.
switch, CDP enables Network Assistant to
display a graphical view of the network. The
switch uses CDP to find cluster candidates
and maintain information about cluster members and other devices up to three
cluster-enabled devices away from the command
switch by default.