CDP Protocol question

[Michael Bartholomæussen]

Hi everybody 

Correct me if I'm worng, but according to Cisco documentation, a switch running CDP will only share information with it's neighbor, meaning that a switch will never pass CDP information on to a second switch.

Today while working on a Nexans switch, it received CDP information from switches 2 and 3 nodes away. It's received CDP information from C,D in a setup with A - B - C - D? Any explanation for this behaviour, or am I missing some information about how CDP operates.

Regards,

Michael

[John Blakley]

Michael,

CDP will go through switches. For example, if I have a Cisco --- Dell ---- Cisco, the Cisco will see the other Cisco, but it won't see the Dell. It's L2, so there's nothing to block the traffic between switches. CDP-supported equipment can read the packet and get the information out of it, whereas non-CDP equipment will happily pass it along....

HTH,

John

[Michael Bartholomæussen]

If the Dell switch in your example is CDP capable, will it store the information and still pass CDP information on to the cisco switch??

Michael

[Rolf Fischer]

Michael,

we also use Nexans FTTO / Harsh Environment switches in some of our Cisco LANs, so I hope you allow me to join.

At least the Nexans products we use support the CDP protocol, so the answer to your question should be "no" in this case. CDP uses the multicast (destination) MAC-address 01-00-0c-cc-cc-cc, which is also used by other Cisco proprietary layer-2 control-plane protocols like VTP, DTP, PAgP and UDLD. If the receiver of such a frame supports the respective protocol, its scope shoud be link-local, in other words: It must not pass the PDU to neighbor switches. If it does not support the protocol, the frame has to be treated like normal layer-2 multicast: Flooding over all ports except the port it was received on.

Could you elaborate a bit on the exact topology (what Cisco devices, what Nexans devices and firmware version, what other 3rd-party devices)?

HTH

Rolf

P.S.: If you're talking about another vendor called Nexans, please let us know.

[Michael Bartholomæussen]

Hi Rolf

The switch that passes on the CDP frame is an iGigaSwitch 160C E+ SFP- 12VI PRO3, firmware is HW3/L-PROF-16PORT/V4.09az and hardware version is V3.31. The switch is CDP capable and there for should not forward CDP frames

Michael

[Rolf Fischer · Accepted solution]

The switch is CDP capable and there for should not forward CDP frames

I agree.

Unfortunately we don't use that switch model and our latest firmware version is HW3/.../4.03cd, so I can only share experiences based on our equipment.

Normally we use only one of their uplinks, but in our lab we also tested designs with redundant uplinks and everything worked as expected. Perhaps you should contact the Nexans support?

HTH

Rolf

[Ganesh Hariharan]

Hi everybody 


Correct me if I'm worng, but according to Cisco documentation, a switch running CDP will only share information with it's neighbor, meaning that a switch will never pass CDP information on to a second switch.

Today while working on a Nexans switch, it received CDP information from switches 2 and 3 nodes away. It's received CDP information from C,D in a setup with A - B - C - D? Any explanation for this behaviour, or am I missing some information about how CDP operates.


Regards,


Michael

Hi Michael,

CDP is a data link layer protocol. It is available in  Routers and catalyst switches support CDP. CDP is enabled by default on all supporting devices.

Supporting device can receive and send CDP messages.

CDP messages are generated as layer two frames and distributed as multicasts. CDP uses SNAP ( Subnetwork Access Protocol) frame type. If all interconnect devices are layer 2 connected i believe that is why cup messages are getting passed from end to end.

Hope it Help..

-GI

Rate if it Helps

[Peter Koltl]

CDP is a data link layer protocol.

CDP is an application protocol above data link layer. (-: