Debug Options for Cisco Discovery Protocol (CDP)

As with all other protocols, Cisco Discovery Protocol (CDP) has specific debug commands that can help you to identify errors or problems with the protocol. You can display debugging information for the following items (as shown by the context help for the command):

Switch1>enable
Switch1#debug cdp ?
  adjacency  CDP neighbor info
  events     CDP events
  ip         CDP ip info
  packets    CDP packet-related information

To see debug cdp in action, the following shows the output for debug cdp events:

Switch1>enable
Switch1#debug cdp events
CDP events debugging is on
Switch1#
CDP-EV:  CDP sending protocol-hello notification
  CDP-EV: protocol-hello notification, contents:
  CDP-EV: notification_count = 1
  CDP-EV: OUI =0x00000C, Protocol ID=0x0112
  CDP-EV: payload len=27, value=00000000FFFFFFFF010121FF0000000000000006D6AC46C0FF0001
  CDP-EV: receiving IDB name = 'FastEthernet0/12'
  CDP-EV: remote device name = 'Switch2', CDP device number = 4
  CDP-EV: remote interface name = 'FastEthernet0/1'
  CDP-EV: src_mac = 0006.D6AC.46C1, capabilities = A
  CDP-EV: duplex = 1, native-vlan = 0
  CDP-EV: received_vtp_mgmt_domain = 1, vtp_mgmt_domain_length = 0
  CDP-EV: vtp-management-domain-name = ''
CDP-EV:  CDP sending protocol-hello notification
  CDP-EV: protocol-hello notification, contents:
  CDP-EV: notification_count = 1
  CDP-EV: OUI =0x00000C, Protocol ID=0x0112
  CDP-EV: payload len=27, value=00000000FFFFFFFF010121FF0000000000000006D6AC46C0FF0001
  CDP-EV: receiving IDB name = 'FastEthernet0/12'
  CDP-EV: remote device name = 'Switch2', CDP device number = 4
  CDP-EV: remote interface name = 'FastEthernet0/1'
  CDP-EV: src_mac = 0006.D6AC.46C1, capabilities = A
  CDP-EV: duplex = 1, native-vlan = 0
  CDP-EV: received_vtp_mgmt_domain = 1, vtp_mgmt_domain_length = 0
  CDP-EV: vtp-management-domain-name = ''
Switch1#no debug all
All possible debugging has been turned off

Scanning through the preceding output, you can identify the following:

  • Switch1 can see Switch2.

  • The interfaces on Switch1 are connected to the interfaces on Switch2.

  • The MAC address of Switch1.

  • The VLAN that the switches are likely using to communicate.

  • The VTP domain to which switches belong.

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