CDP help

thaershunnar · ‎07-11-2007

hi,

the following appears when i apply the sho cdp command on a cisco switch, can any one help me to understand what does it mean when the cdp said the port Gig0/1 is connected to mor than a device ??

EngB1140S0#sho cdp ne

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

unkown2222 Gig 0/1 133 S I WS-C2950T-Gig 0/1

JAE070923AQ Gig 0/1 143 T S WS-C4003 2/1

unknown1111 Gig 0/1 152 S I WS-C2950T-Fas 0/21

bill.efthimiou · ‎07-11-2007

CDP uses multicast (layer 2 from memory) to advertise it's presence and detail that you normally see in show cdp output.

These CDP multicast packets are "ABSORBED" by neighbouring "CISCO" devices that themselves understand CDP. They absorb and process teh multicast packet to be able to build the CDP table. However, if you place a NON CISCO device between two Cisco devices, the NON Cisco device will NOT absorb this CDP multicast traffic, because it does not understand CDP. The Non Cisco device will forward/flood the multicast traffic.

This is what I believe is probably happening in your situation.

That is, your device "EngB1140S0", is connected to a non cisco device or a hub on port G0/1. This non Cisco device then has the three listed devices connected somewhere behind it. Either directly connected or further down stream, I can't tell from your output. These three devices are all sending CDP multicast packets. The non Cisco device is receiving the CDP traffic from all three devices and flooding out the all ports including G0/1, which is EngB1140S0. Because you receive CDP packets for these three devices on G0/1, it gives the appearance they they are all directly connected to G0/1.

I hope this is clear enough and help you.

Regards,

Bill.