Examining Cisco Discovery Protocol (CDP) Output

Once you have Cisco Discovery Protocol (CDP) running and collecting data about your neighboring devices, you may want to view its output. This information about neighboring devices, is obtained by using the show cdp neighbors command.

The following is a list of options that go with that command. You can be specific about which neighbors you want to see, or you can show them all, and you can view summary data or detailed data.

Switch1>enable
Switch1#show cdp neighbors ?
  Async              Async interface
  BVI                Bridge-Group Virtual Interface
  Dialer             Dialer interface
  FastEthernet       FastEthernet IEEE 802.3
  Lex                Lex interface
  Multilink          Multilink-group interface
  Port-channel       Ethernet Channel of interfaces
  Tunnel             Tunnel interface
  Virtual-Template   Virtual Template interface
  Virtual-TokenRing  Virtual TokenRing
  Vlan               Catalyst Vlans
  detail             Show detailed information
  |                  Output modifiers
  <cr>

You can get detailed data with the cdp command. Here is detailed information that can be seen about my neighboring devices.

Notice the device type of RootBridge.edtetz.net; pick out which switch port can see Router2; and decode what IOS version is running on Switch2.

Switch1>enable
Switch1#show cdp neighbors detail
-------------------------
Device ID: Switch2
Entry address(es):
  IP address: 192.168.1.243
Platform: cisco WS-C2950-12,  Capabilities: Trans-Bridge Switch
Interface: FastEthernet0/12,  Port ID (outgoing port): FastEthernet0/1
Holdtime : 137 sec
Version :
Cisco Internetwork Operating System Software
IOS (tm) C2950 Software (C2950-C3H2S-M), Version 12.0(5.3)WC(1), MAINTENANCE INTERIM SOFTWARE
Copyright (c) 1986-2001 by cisco Systems, Inc.
Compiled Mon 30-Apr-01 07:56 by devgoyal
advertisement version: 2
Protocol Hello:  OUI=0x00000C, Protocol ID=0x0112; payload len=27, value=00000000FFFFFFFF010121FF0000000000000006D6AC46C0FF0001
VTP Management Domain: ''
Management address(es):
-------------------------
Device ID: Router2
Entry address(es):
  IP address: 192.168.1.240
Platform: Cisco 2621XM,  Capabilities: Switch IGMP
Interface: FastEthernet0/3,  Port ID (outgoing port): FastEthernet0/0
Holdtime : 142 sec
Version :
Cisco IOS Software, C2600 Software (C2600-ADVIPSERVICESK9-M), Version 12.3(4)T4,  RELEASE SOFTWARE (fc2)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2004 by Cisco Systems, Inc.
Compiled Thu 11-Mar-04 19:57 by eaarmas
advertisement version: 2
VTP Management Domain: ''
Duplex: full
Management address(es):
-------------------------
Device ID: RootBridge.edtetz.net
Entry address(es):
  IP address: 192.168.1.103
Platform: AIR-AP350,  Capabilities:
Interface: FastEthernet0/1,  Port ID (outgoing port): fec0
Holdtime : 131 sec
Version :
Cisco 350 Series AP 12.03T
advertisement version: 2
Duplex: full
Power drawn: 6.000 Watts
Management address(es):

After reviewing the code, you should now know

  • RootBridge.edtetz.net is an AIR-AP350 — an Aironet 350 series access point

  • Router2 is seen by port FastEthernet0/3 — Router2 is using FastEthernet0/0

  • Switch2 is running IOS version 12.0

The show command offers a long list of items you can see information about. One of those items is cdp interface. The show cdp interface command provides you with detailed information about the interfaces CDP is running on. The output has been shortened to show only the first four switch interfaces. Notice that all the interfaces are still running at the default CDP configuration.

Switch1>enable
Switch1#show cdp interface
FastEthernet0/1 is up, line protocol is up
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
FastEthernet0/2 is up, line protocol is up
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
FastEthernet0/3 is up, line protocol is up
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
FastEthernet0/4 is down, line protocol is down
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
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