CDP neighbours showing self Strange - Inherited poor deployed Network

[Zulfikri Yahya]

Hi Team, 

One of my field engineers at a remote site has reported a problem to me, one 
which I find rather strange but yet to confirm for myself. 

A C4507R  at the remote site is linked to another core C4507R without VRRP or any load balance mechanism.

Am just trying to do a mapping as we just been handed this network segment without any single documentation or anything. (tough)

When I issue "SHOW CDP NEI" command on the remote C2960 switch, I see the switch itself appearing as its 
neighbor with some other neighbors.

89-CSW-4001#sh cdp neighbors 
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
FPSO Cendor Wireless Controller
                 Gig 1/35          174               H    AIR-CT250 Gig 0/0/1
CP2_AP_03        Gig 1/42          134              R T   AIR-CAP26 Gig 0.1
CP2_AP_04        Gig 1/42          167              R T   AIR-CAP26 Gig 0.1
89-CSW-4002      Gig 2/13          138             R S I  WS-C4507R Gig 1/8
89-CSW-4001      Gig 1/40          133             R S I  WS-C4507R Gig 2/38
89-CSW-4001      Gig 2/38          133             R S I  WS-C4507R Gig 1/40
89-LSW-4401      Gig 2/42          130              S I   WS-C2960- Gig 0/2
89-LSW-4101      Gig 2/45          163              S I   WS-C2960- Gig 0/2

I have personally never seen a Cisco device showing itself as it's own 
neighbor in a 'show cdp neighbor' output. 

What do you know to be the cause of this problem? Looping? most probably?

Thanks. 

[Leo Laohoo]

I would run a trace.  It looks like a loop.  Ports might be an 802.1Q trunk. 

[ranjipremandrewp]

Leo & Madhu,

Does this mean that the ports Gig 2/38 and Gig 1/40 are interconnected?

89-CSW-4001      Gig 1/40          133             R S I  WS-C4507R Gig 2/38
89-CSW-4001      Gig 2/38          133             R S I  WS-C4507R Gig 1/40

[Leo Laohoo]

Does this mean that the ports Gig 2/38 and Gig 1/40 are interconnected?

Yes.  Most likely through a transparent bridge or a hub.

[Madhukrishnan Gopinathan Nair]

In a maintenance window, shut one of them over a CONSOLE connection, and check the other interface still UP/UP,then it means there is a device in between.

Madhu

[Madhukrishnan Gopinathan Nair]

what is 89-csw-4001 a 4500 or 2960?

Anyway please check the cable to ensure that there is no other devices in between.

Thanks,

Madhu.

[Zulfikri Yahya]

Hi Madhu,

It's another 4500. Hard to trace the cabling as it goues unto the wall and no tag. No devices in between.

cheers

regards,

Zulfikri

[c39213679]

I had the same issue and found that two ports from my Cisco 9200 switch were plugged into an unmanaged non Cisco switch.  The unmanaged switch was able to participate in Rapid Spanning Tree, so my switch blocked one of the ports as expected.  However, it looks like CDP frames from my switch are being broadcast (reflected back) by the unmanaged switch.

[Giuseppe Larosa]

Hello @c39213679 ,

the non Cisco switch forwards the CDP frames treating them as L2 multicast frames, because they are frames with a multicast MAC address destination.

So what you see is expected

Hope to help

Giuseppe

[Richard Burts]

I believe that Giuseppe is spot on with his explanation. Your switch sends CDP frames with multicast destination mac address. When this is received by a Cisco device it recognizes the CDP frame, processes it, and does not forward it. But a non Cisco device sees an incoming frame with multicast destination and forwards the frame.