CDP - multiple devices connected to a single interface

[c_hullah1]

I am currently carrying out an audit for a remote site and building up the network topolgy by using CDP to show the neighbours however I am puzzled with the below results which I have found on several switches.

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID

XXXXXXXX          Gig 0/24            153              T S           1900         A

XXXXXXXX          Gig 0/24            145              T S           1900         A

XXXXXXXX          Gig 0/24            155              T S           1900         A

The switch that I am connected to seems to have several devices connected to it via a single port (Gi0/24) which is something I have never seen before.

Port Gi0/24 is configured as a trunk allowing all VLANs:

Gi0/24                       connected    trunk      a-full a-1000 1000BaseSX SFP

I suppose a hub connecting these devices could explain the matter but the connected devices are apparently in different buildings.

Also the Port ID is A which I can't find any reference to online.

I'd be grateful for any suggestions as to what could cause these entries to appear and what the port ID A means.

Many thanks
Chris

[Jeff Van Houten]

Gi0/24 is a fiber connection (1000 base sx) to another switch.

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[Giuseppe Larosa · Accepted solution]

Hello Chris,

>> I suppose a hub connecting these devices could explain the matter but the connected devices are apparently in different buildings.

Be aware that CDP is Cisco proprietary so also a non Cisco LAN switch in the middle can explain this. For a non Cisco LAN switch CDP frames are simply multicast user traffic to be propagated everywhere within the Vlan.

>> XXXXXXXX          Gig 0/24            153              T S           1900         A

Catalyst C1900 are quite old and if I remember correctly their uplink ports had names like A or B. Some C1900 have an IOS CLI other ones are menu based depending on the software version.

It looks like there is an access layer built with C1900 devices and aggregated on an non Cisco LAN switch

Hope to help

Giuseppe

[c_hullah1]

Thank you for the reply, some very good points there, there may well be some non cisco hardware on the site, I will investigate.

Thanks
Chris

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