CDP – Cisco Discovery Protocol

CDP – Cisco Discovery Protocol

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CDP is a Cisco proprietary protocol used to collect information from devices on the network that is directly connected, such as hardware, software, device name and more.

CDP, a Cisco proprietary protocol, is not compatible with other brands, operates at layer 2 (data link), and is enabled by default.

CDP is currently in version 2:

  • V1:- This version only collected information from the neighboring device.
  • V2:- With this version CDP included improvements, CDP can identify a mismatch with the native vlan IDs or a mismatch in the duplex of the devices that are directly connected to it.

Command to view the version, announcement interval, and holdtime of CDP:

Switch#show cdp           
Global CDP information:
        Sending CDP packets every 60 seconds
        Sending a holdtime value of 180 seconds
        Sending CDPv2 advertisements is  enabled
Switch#

As seen in the previous output, CDP advertisements are sent every 60 seconds by default, which are sent for each active interface. If after 180 seconds the switch does not receive CDP advertisements for any interface, it proceeds to delete the neighbor.

Command to show the information about the CDP announcements that a switch has received:

Switch# show cdp neighbors [ type member/module/number] [ detail]

Switch# show cdp neighbors gig1/0/24
Switch#
Switch# show cdp neighbors 
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
SW5              Fas 1/4            166          S I      WS-C3750G-Gig 1/0/13
Switch03         Gig 2/1            139          S I      WS-C3750G-Gig 1/0/85
SW6              Fas 5/10           142          S I      WS-C3750G-Gig 1/0/18
FA3A55D          Fas 0/9            132          H P      IP Phone 7 Port 1
SW7              Fas 0/5            162          S I      WS-C3750G-Gig 1/0/11
SWVLAN           Fas 0/3            166          S I      WS-C3750G-Gig 1/0/19
SW1              Fas 0/8            155          S I      WS-C3750G-Gig 1/0/16
SW2              Gig 1/40           140          S I      WS-C3750G-Gig 1/0/12

Switch#

Add the keyword detail to view all CDP information received from an advertisement.

Switch# show cdp neighbors detail

The above command will display the detailed information of all known devices through CDP. We can filter the information through a single interface:

Switch# show cdp neighbors Fas 1/1 det
-------------------------
Device ID: Sw01
Entry address(es): 
  IP address: 10.2.6.4
Platform: cisco WS-C3750G-24TS-1U,  Capabilities: Switch IGMP 
Interface: FastEthernet1/1,  Port ID (outgoing port): GigabitEthernet2/0/24
Holdtime : 169 sec

Version :
Cisco IOS Software, C3750 Software (C3750-ADVIPSERVICESK9-M), Version 12.2(44)SE6, RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2009 by Cisco Systems, Inc.
Compiled Mon 09-Mar-09 17:56 by gereddy

advertisement version: 2
Protocol Hello:  OUI=0x00000C, Protocol ID=0x0112; payload len=27, value=00000000FFFFFFFF010221FF000000000000FCFBFB780E00FF0000
VTP Management Domain: ''
Native VLAN: 1
Duplex: full

Switch#

To disable CDP globally it is done with the following command:

!
Switch(config)# no cdp run
!

Sometimes, for security reasons, you may want to disable CDP announcements on an individual interface and/or interfaces that are not connected to Cisco equipment:

!
Switch(config)# int fa0/12
Switch(config-if)# [no] cdp enable
!

In summary, here is a list of commands to configure CDP:

(config)# cdp run                     Enables CDP globally on device.
(config)# no cdp run                  Disables CDP globally on device.
(config-if)# cdp enable               Enables CDP on an interface device if CDP is enabled globally.
(config-if)# no cdp enable            Disables CDP on an interface device.
(config)# cdp timer <seconds>    
(config)# cdp holdtime <seconds>

Verifications commands:

!
Switch# clear cdp counters
Switch# clear cdp table
Switch# show cdp
Switch# show cdp entry device-name [protocol | version]
Switch# show cdp interface [type number]
Switch# show cdp neighbors [type number] [detail]
Switch# show cdp traffic
!

More information:

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/15-2_4_e/configurationguide/b_1524e_consolidated_3750x_3560x_cg/b_1524e_consolidated_3750x_3560x_cg_chapter_0100101.html

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