Question

No, CDP is layer 2.

Steve

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

"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

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

No. L3 is network layer, so you would need an appropriate L3 protocol configured correctly for it to work.