sh cdp neighbors port-channel command

[Joris Deprouw]

Hello,

I have a strange issue with the command sh cdp neighbors port-channel on my 3750 and 4500switches.

When I execute the command "sh cdp neighbors port-channel". I do not see my neigbors on this interface.

The command "sh cdp nei" with the port-channel works fine and shows all my neighbors.

switch#sh cdp neighbors port-channel 1

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,
                  D - Remote, C - CVTA, M - Two-port Mac Relay

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID

switch#sh ether summary
Flags:  D - down        P - bundled in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        U - in use      f - failed to allocate aggregator

        M - not in use, minimum links not met
        u - unsuitable for bundling
        w - waiting to be aggregated
        d - default port

Number of channel-groups in use: 1

Number of aggregators:           2

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         LACP      Gi1/0/1(P) Gi1/0/2P)

I tried the command on several IOS versions, but always the same issue. When I try the command on a 6506-E it works fine...

Is this command platform related?

switch2# sh etherchannel summary
Flags:  D - down        P - bundled in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        U - in use      N - not in use, no aggregation
        f - failed to allocate aggregator

        M - not in use, no aggregation due to minimum links not met
        m - not in use, port not aggregated due to minimum links not met
        u - unsuitable for bundling
        d - default port

        w - waiting to be aggregated
Number of channel-groups in use: 7
Number of aggregators:           7

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         LACP      Gi1/1(P)       Gi1/2(P)
...

switch2#sh cdp nei port-channel 1
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
switch             Gig 1/2           123           S I                          Gig 1/0/1
switch             Gig 1/1           123           S I                          Gig 1/0/2

Thanks,

best Regards,

Joris

[Shashank Singh · Accepted solution]

Hi Joris,

This is a known issue and is documented as an internal cosmetic bug. Logically speaking, CDP is not supported on etherchannel interfaces and "sh cdp port-channel" command should not be available.

On 6500 platform, though the "show cdp neighbor port-channel" returns the output, it is just for the sake of convenience and does not mean that CDP is supported on port channels. In fact if you try enabling CDP on port-channel on 6500, it throws an error:

6500(config)#int po1

6500(config-if)#cdp en

% CDP is not supported on this interface, or for this encapsulation

If you run 'debug cdp events' you can see the packets being exchanged on the physical interface rather than on the port-channel. Correct way is to check the cdp neighborship on physical interfaces.

Cheers,

Shashank

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[Joris Deprouw]

Thanks ShaShank,

I suspected this. But strange that it is not fixed in the newest IOS versions...

Best Regards,

Joris