If LLDP is supported on physical interfaces, what about CDP?

[MikeNY85]

I have been reading about LLDP and CDP. I absolutely understand that CDP is a Cisco proprietary protocol which runs on Cisco machines. I was reading about LLDP and how it's different from CDP. I am a bit confused now with the fact that LLDP is supported on physical interfaces and  only discovers one device per port. If that's the case with LLDP, so what are the kinds of interfaces that CDP supports?. Doesn't CDP support one device per port? the same as LLDP?.

Any clarification from the professionals would be much appreciated!

Thank you!

[Richard Burts · Accepted solution]

It is not necessarily true that CDP is limited to a single device per port. Let us think about this environment: a Cisco device is connected to a non Cisco switch Ethernet port. Also connected to that non Cisco switch are 4 other Cisco devices. The CDP frames sent by the other devices are forwarded by the switch and the original Cisco device will report 4 CDP neighbors on the single interface.

[MikeNY85]

Thanks for the answer and I really appreciate it. But isn't CDP supposed to learn about only directly connected devices? (not devices connected to a device that is connected to a switch of example)?. I absolutely not saying you are wrong, but this is what i saw in the book. Also what is it about LLDP supporting physical interfaces only, unlike CDP, If you can explain that to me I would appreciate it!.

Thanks again for you help!

[Richard Burts · Accepted solution]

You ask "But isn't CDP supposed to learn about only directly connected devices?" and the answer is no it is frequently but not always directly connected devices. MHM has supplied an example where a not directly connected device is discovered. And the example that I gave I have observed in real live networks. Perhaps it might help to think about it in this way: CDP on an Ethernet interface sends out a frame with a destination mac address that identifies it as CDP traffic. If the original device is connected to a Cisco switch then the switch recognizes that it is CDP, processes the frame, and does not forward that frame out other interfaces. But if the original device is connected to a non Cisco switch then that switch receives the CDP frame and forwards it out its other interfaces. So if there are 4 other Cisco devices connected on that switch then CDP will discover 4 neighbors on that single interface.

I do not know what you read in that book and so do not know whether this is perhaps the book was describing a "usual" behavior and you interpreted it as an "always" behavior, or whether the book might contain an error (which has been known to happen). But I can assure you that CDP is not limited to directly connected devices.

[MikeNY85]

Thanks for your answer, and sorry for the confusion. Your help is much much appreciated!

Thanks again!

[Richard Burts]

You are welcome. I am glad that our explanations have been helpful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.

[MHM Cisco World · Accepted solution]

the answer is in VoIP network, 
you connect PC to Iphone and then connect iphone to SW, the Iphone will send two CDP to SW
one for it and other for PC connect to it
that what meaning of direct and indirect connection device to SW.

[MHM Cisco World · Accepted solution]

friends, CDP is L2 protocol, and if SW not know this protocol it will drop it, it never forward it.
this what I know. 

[Defututus]

Its L2 multicast. It will be forwarded/flooded if the switch does not support cdp

[Joseph W. Doherty · Accepted solution]

FYI, BTW, CDP will even run across GRE tunnels.