Protocol is a Layer 2, media-independent, and network-independent protocol that
networking applications use to learn about nearby, directly connected devices.
Cisco Discovery Protocol is enabled by default. Each device configured for
Cisco Discovery Protocol advertises at least one address at which the device
can receive messages and sends periodic advertisements (messages) to the
well-known multicast address 01:00:0C:CC:CC:CC. Devices discover each other by
listening at that address. They also listen to messages to learn when
interfaces on other devices are up or go down.
contain time-to-live information, which indicates the length of time a
receiving device should hold Cisco Discovery Protocol information before
discarding it. Advertisements supported and configured in Cisco software are
sent, by default, every 60 seconds on interfaces that support Subnetwork Access
Protocol (SNAP) headers. Cisco devices never forward Cisco Discovery Protocol
packets. Cisco devices that support Cisco Discovery Protocol store the
information received in a table. Information in this table is refreshed every
time an advertisement is received, and information about a device is discarded
after three advertisements from that device are missed.
contained in Cisco Discovery Protocol advertisements varies based on the type
of device and the installed version of the operating system. Some of the
information that Cisco Discovery Protocol can learn includes:
Cisco IOS version
running on Cisco devices
IP addresses of
interfaces on devices
devices advertising Cisco Discovery Protocol
on Cisco devices, including encapsulation type
Protocol (VTP) domain
Protocol Version 2 provides more intelligent, device-tracking features than
those available in Version 1. One of the features available is an enhanced
reporting mechanism for more rapid error tracking, which helps to reduce
network downtime. Errors reported include mismatched native VLAN IDs (IEEE
802.1Q) on connected ports and mismatched port-duplex states between connected
devices. Messages about reported errors can be sent to the console or to a
You can use
for getting detailed output on VTP management domains and duplex modes of
neighboring devices, counters related to Cisco Discovery Protocol, and VLAN IDs
of connecting ports.
When one interface
is configured or PPP is configured alone then the CDP configuration is removed
with the removal of configuration of interface or PPP and the CDP process is
disabled. In case, if there are two interfaces configured and configuration of
one is removed then the CDP configuration is retained with that interface.
Using Cisco Discovery Protocol with SNMP
Using Cisco Discovery Protocol with Simple Network Management Protocol (SNMP) allows network management applications to learn the type of device and the SNMP agent address of neighboring devices. Applications can also send SNMP queries to these neighboring devices.
The SNMP management application learns protocol addresses and types of neighboring devices by retrieving the Cisco Discovery Protocol tables from SNMP agents on those devices. When enabled, the network management module (NMM) SNMP agent discovers neighboring devices and builds its local cache with information about these devices. A management workstation can retrieve this cache by sending SNMP requests to access CISCO-CDP-MIB.
Cisco Discovery Protocol and On-Demand Routing Support for ATM PVCs
Cisco Discovery Protocol and On-Demand Routing (ODR) support exists for ATM point-to-point permanent virtual circuits (PVCs). ODR uses Cisco Discovery Protocol to propagate IP address information in hub-and-spoke topologies. When ODR is enabled, spoke routers automatically advertise their subnets by using Cisco Discovery Protocol.
Cisco Discovery Protocol is disabled by default on ATM PVC interfaces. To enable Cisco Discovery Protocol, use the
run command in global configuration mode and the
enable command in interface configuration mode on both ends of the PVC. To enable ODR, use the
router odr command in global configuration mode on the hub router, and turn off all dynamic-routing protocols on the spoke routers. For details about configuring ODR, see the “Configuring On-Demand Routing” section in the
IP Routing: ODR Configuration Guide.