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Cisco Discovery Protocol (CDP)
c2500 Cat3550 Cat2900XL
E0 Fa0/1 Fa0/2 Fa0/1
192.168.0.1 192.168.0.2 192.168.0.3
Figure 10-1: Network Setup for Cisco Discovery Protocol
- Cisco Discovery Protocol (CDP) is a Cisco-proprietary data link layer device discovery
protocol designed for gathering hardware and software information of neighboring devices. Such
info is useful for troubleshooting and documenting a network.
- CDP updates are sent as multicasts. It is a L2 protocol and runs on all physical interfaces that
support SNAP (Subnetwork Access Protocol), eg: all LAN protocols, HDLC, Frame Relay, and
ATM. The only media type CDP cannot operate over is X.25, as it does not support SNAP.
- CDP is a L2 protocol, hence it is independent on any L3 protocol – Cisco devices are able to
discover L3 addressing details of neighboring devices even without any network layer (L3)
addressing configuration.
- The show cdp neighbors detail EXEC displays many useful information regarding all
neighboring devices.
- CDP frames do not pass through switches, they are only sent to directly connected devices.
Ex: In Figure 10-1, c2500’s CDP neighbor is only Cat3550; Cat2900XL is not its CDP neighbor.
- Below interprets the fields of the output of the show cdp neighbors EXEC command:
Field Description
Device ID The hostname of the directly connected device.
Local Interface The interface of the local device that received the CDP packet from the
directly connected device.
Holdtime The amount of time a device will hold (keep) the received CDP information
before discarding it if no more CDP packets are received.
Capability The capabilities of the directly connected device, eg: router, switch, etc.
The device capability codes are listed at the top of the command output.
Platform The model of the directly connected device.
Port ID The interface of the neighboring device that transmitted the CDP packet.
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192.168.0.11
Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com