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Switch shows itself on cdp


I have 3 348G switches connected in eiasy chain fashion on gig0/0 and gig0/1 ports. My problem is that when I do "show cdp neighbor" command, it show switch seeing itself on some of fastethernet ports.  fastethernet ports are all connected PCs. the closet is wired with old type 1 cable for token ring, but later conneverted into ethernet.

Please if any has anybody have a clue of this, let me know. 

thank you.

WITCHNGS2   Fas 0/24           173          S I      WS-C2950G-Fas 0/24
WITCHNGS2   Fas 0/15           173          S I      WS-C2950G-Fas 0/15
SWITCHNGS2   Fas 0/9            173          S I      WS-C2950G-Fas 0/9
WITCHNGS2   Fas 0/8            173          S I      WS-C2950G-Fas 0/8

Peter Paluch
Hall of Fame Cisco Employee


I would say that the most reasonable explanation is that the CDP frames are somehow "reflected" back. Are you absolutely sure that on these ports (Fa0/24, Fa0/15, Fa0/8), only end hosts are connected? Is it a direct connection by cable only, or is there any device between the port and the end host, no matter how insignificant it may seem?

Best regards,


thanks Peter,

The only thing that is in between switch ports and end hosts  is type 1 batch panel.      Type 1 cables run from cubes to the patch panel in closet, then token ring to Ethernet convertor adapters and cat 5 cables are used between switch ports to patch panel.

I'm suspecting the type 1 patch panel, but then it is not happening on ports.

thank you.

You are correct

The problem you see is due to the type 1 cabling... we have seen this happen with patch panels that convert type1 to ethernet.

when the workstation is disconnected the tran/receive pairs are looped causing the switch to see a loop.

The type 1 cables were designed for token ring so when the cable is disconnected at the host end, it forms a loop restoring the ring



Peter Paluch
Hall of Fame Cisco Employee


That would make sense. I have no practical experience with Token Ring equipment - thank you for joining in!

Nevertheless, something here confuses me: On Catalyst switches, every switchport emits LOOP frames until explicitly disabled. If a port receives a LOOP frame it has originated itself, it will be immediately put into self-looped errdisabled state. Why did not that happen in this network?

Best regards,



   We have seen this when they use gigastack gbics and you tie them all together . Ugly setup and tough to troubleshoot and find anything including tracking down addresses .  I bet if you look closely they are using this setup .

Thank you all,

some of the switches use gigastack gbics, but this issue is also happening on those switches not using gigastack  gbics.  Narayan  comments makes more sense. If I look those ports closely, I don't find  mac addresses associated with them, yet they show line up protocol up. I'm also seeing BPDU Guard triggered on end host ports randamly and I did not find any device capable of sending BPDU, so most likely the type 1 cables and patch panel are causing loops.