Question

No, CDP is layer 2.

Steve
www.networking-forum.com

Can you explain that ?
Physical/Data = the Network interface
Network = Internetwork
Transport = Transport
Session/Presentation/Application = Application

I don't see why CDP should be in layer 2.

From
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2950/12113ea1/2950scg/swcdp.htm

"CDP is a device discovery protocol that runs over Layer 2 (the data link
layer) on all Cisco-manufactured devices (routers, bridges, access servers,
and switches) and allows network management applications to discover Cisco
devices that are neighbors of already known devices. With CDP, network
management applications can learn the device type and the Simple Network
Management Protocol (SNMP) agent address of neighboring devices running
lower-layer, transparent protocols. This feature enables applications to
send SNMP queries to neighboring devices.

CDP runs on all media that support Subnetwork Access Protocol (SNAP).
Because CDP runs over the data-link layer only, two systems that support
different network-layer protocols can learn about each other.

Each CDP-configured device sends periodic messages to a multicast address,
advertising at least one address at which it can receive SNMP messages. The
advertisements also contain time-to-live, or holdtime information, which is
the length of time a receiving device holds CDP information before
discarding it. Each device also listens to the messages sent by other
devices to learn about neighboring devices. "

Hopefully this helps.

Regards,

Steve

www.networking-forum.com

Thanks, this is really usefull... I had this question on my CCNA and I gave the
wrong answer.

Seems CDP is a special case, of course because it's Cisco own thing.
I use it a lot in my daily work but never knew it was running on L2.

thanks steve



http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2950/12113ea1/2950scg/swcdp.htm

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2950/12113ea1/2950scg/swcdp.htm

Hans wrote:



No. L3 is network layer, so you would need an appropriate L3 protocol
configured correctly for it to work.
--
Paul Matthews                                         CCIE #4063
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