In this lesson, we will discuss the Layer 2 discovery protocol.

1. CDP (Cisco Discovery Protocols)

2. LLDP (Link Layer Discovery Protocol)

The main function of these protocols is to collect and share information with neighboring devices. Shared information includes host name, IP address, and device type, etc. When a device receives a CDP/LLDP message, it adds the neighbor information to the neighbor table and discards the message. It doesn’t forward the message to other devices. So, this way, you can see that only directly connected devices can become CDP/LLDP neighbors.

CDP (Cisco Discovery Protocol)

It is a Cisco proprietary protocol. By default, CDP is enabled on all Cisco devices (routers, switches, firewalls).
CDP message timer = 60 sec, CDP holddown timer = 180 sec
After sending the CDP message, it waits for 180 seconds. If there is no response within this time from the neighbor side, then it deletes the neighbor from the neighbor table.
CDP messages are periodically sent (after 60 seconds) to multicast address 0100.0ccc.cccc

By default, CDP is enabled on the device. To disable the CDP, use no cdp run command in global configuration mode.

R1(config) # no cdp run     -—to disable the cdp

R1(config) # cdp run           -—to enable the cdp

By default, CDP is enabled on each interface. To disable CDP, use the no cdp enable command in interface mode.

R1(config-if) no cdp enable  —-— to disable the cdp

R1(config-if) cdp enable  —-— to enable the cdp

R1# show cdp neighbors  –— this command will show a list of LLDP neighbors and some basic information.

CDP neighbor table

LLDP (Link Layer Discovery Protocol)

It is an industry standard protocol. By default, LLDP is disabled on all devices.
LLDP message timer = 30 sec and LLDP hold-down timer =120 sec.
LLDP messages use multicast MAC address = 0180.C200.000E.
A device can run CDP and LLDP at the same time.

Here are some useful commands-

1. R1(config)# lldp run   –— to enable the lldp on the device.

2. R1(config)# no lldp run   –— to disable the lldp on device.

3. R1(config-if)# lldp transmit  –— allow interface to transmit lldp messages.

4. R1(config-if)# lldp receive –— allow interface to receive lldp messages.

5. R1# show lldp neighbors  –— this command will show a list of lldp neighbors and some basic information.