need help to understand CDP

Answered Question
Jul 11th, 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 ??

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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

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Correct Answer by mheusing about 9 years 5 months ago

Hi,

CDP messages are sent as Layer2 frames to a specific MAC address reserved for CDP known to all Cisco devices. If a cisco device receives a CDP packet it will process it locally and not forward it. This does not happen for any other vendors switch not supporting CDP. For those switches a CDP packet is just an unknown MAC address, as it will never be used as a source MAC. The natural treatment for unknown frames is to forward them out through all ports belonging to the VLAN, where it was received.

Writing all this, the natural explanation to the observed "show cdp nei" output is:

Your topology consists of 4 Catalyst switches (2 C2950T, one C4003 and the switch you executed the command on) interconnected by a non-Cisco switch. The non-cisco switch will forward all received CDP frames out all ports and thus on Gig0/1 you receive CDP frames from more than one device.

Hope this helps!

Regards, Martin

P.S.: If your topology is completely different then please point out what it looks like.

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Correct Answer
mheusing Wed, 07/11/2007 - 03:03

Hi,

CDP messages are sent as Layer2 frames to a specific MAC address reserved for CDP known to all Cisco devices. If a cisco device receives a CDP packet it will process it locally and not forward it. This does not happen for any other vendors switch not supporting CDP. For those switches a CDP packet is just an unknown MAC address, as it will never be used as a source MAC. The natural treatment for unknown frames is to forward them out through all ports belonging to the VLAN, where it was received.

Writing all this, the natural explanation to the observed "show cdp nei" output is:

Your topology consists of 4 Catalyst switches (2 C2950T, one C4003 and the switch you executed the command on) interconnected by a non-Cisco switch. The non-cisco switch will forward all received CDP frames out all ports and thus on Gig0/1 you receive CDP frames from more than one device.

Hope this helps!

Regards, Martin

P.S.: If your topology is completely different then please point out what it looks like.

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