Cisco Discovery Protocol (CDP) overview

Cisco Discovery Protocol (CDP) is a proprietary protocol developed by Cisco to discover information about locally attached Cisco devices. With CDP, the administrator can collect computer and protocol information about neighboring devices, which can be helpful in troubleshooting or documenting the network.

To discover information, Cisco devices send each of their interfaces with CDP messages. These messages contain information about them, such as Host names, network and data link addresses, device model, IOS version, etc.

To view information about online devices, use the show cdp neighbor command:

Floor 1 # shows cdp neighbors

Capacity codes: R router, T-Trans bridge, B source source bridge

S - switch, H - host, I - IGMP, r - repeater, P - phone

Device ID Port ID of the local Intrfce Holdtme capability

Gig 0/0 166 S 2960 Phase 0/1 change

As you can see in the above example, there is a directly connected device. Here is a description of each field:

Device ID Host name of the online device. In this case, the host name is Switch.
Local interface - the local interface on which the CDP messages were received (in this case Gi0 / 0).
Hold time - how long the local unit stores the information before it is discarded if no more CDP packets are received.
Option - The function of the online device. The letter S indicates that the online device is a switch. The letter R should indicate a router.
Platform model and operating system level running on the neighbor, the 2960 series switch in this case.
Port ID - The interface of the neighboring device to which CDP packets were sent, in this case Fa0 / 1.

For more information about the neighbors, use the show cdp neighborhood details command:

Floor1 # shows the details of the CDP neighbors

Device ID: Change
Address (s):
Platform: Cisco 2960, Features: Switch
Interface: GigabitEthernet0 / 0, port ID (outgoing port): FastEthernet0 / 1
Waiting time: 126

Cisco IOS Software, C2960 Software (C2960-LANBASE-M), Version 12.2 (25) FX, RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2005 by Cisco Systems, Inc.
Composed Wed 12-Oct-05 22:05 by pt_team

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