Layer 2 Discovery Protocols

In this section, we will discuss two popular layer 2 protocols that help us as networking professionals to map a network topology without seeing a network diagram. At the end of this topic, you'll be able to determine the roles, local interfaces, model numbers, and even IP addresses of directly connected neighbor devices while having a clear idea of the actual network topology.

The following exercises are executed in our existing VLAN topology lab.

Cisco Discovery Protocol (CDP)

CDP is a Cisco proprietary protocol that operates at layer 2, the data link layer. CDP is used to assist Cisco devices to learn about their directly connected neighbors, such as other switches and routers. CDP is enabled by default on Cisco switches and routers.

Important note

Devices exchange advertisements (messages) using a multicast address, 01:00:0C:CC:CC:CC.

A CDP message contains the following:

• The IOS version
• The device model and type
...