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New Member

How CDP Works

Hi, want to get more informaiton about workings of CDP. In particular about: 1) why CDP needs SNAP encapsualtion 2) does Frame Relay run using SNAP? 3) Some text says ATM does NOT support CDP some say it does?

If someone enlighten me on this or refer to a decent documentatio on this topic, please. thanks Tariq

New Member

Re: How CDP Works


CDP runs on all media that support SNAP, including Ethernet, Frame Relay, and ATM physical media. CDP runs over the data link layer only. Therefore, two systems that support different network-layer protocols can learn about each other.

I would say the main reason SNAP is needed is because of the spread of different media that support it seems to be more of an interoperability issue.

New Member

Re: How CDP Works

Hi Adam

Many thanks for that. What I can see from some documents:

1) SNAP ecapsulation is what CDP frames are enveloped in. So if the configured connection such as Frame Relay can carry frame then CDP would work.

2) Not all models of ATM devices and all versions of IOS support CDP on ATM interfaces. It seems high end ATM routers with IOS 12.2 can support CDP.

Another question is if CDP is supported on SONET/SDH?

What I can't find is a more detailed document on CDP... Any ideas?

Thanks again.


Re: How CDP Works

of course there is the cisco provided info on CDP as linked below:

here are a couple more links with the information displayed nicely:


Re: How CDP Works

Hi ,

CDP support is introduced in Cisco IOS Software Release 12.2(8)T. Currently, CDP is only supported on AAL5SNAP RFC 1483 PVCs and only on point-to-point subinterfaces. Multipoint subinterface support is not planned as yet.

Below link gives good example for ATM.