Layer 2 Discovery Protocols

Layer 2 discovery protocols (CDP and LLDP) enable network devices to share and discover information about their neighboring (directly connected) devices.

This information includes:

• Host name
• IP address
• Device type

Note: These protocols can pose potential security risks if not properly managed.

CDP (Cisco Discovery Protocol)

CDP is Cisco's proprietary discovery protocol that comes enabled by default on Cisco network devices, including:

• Routers
• Switches
• Firewalls
• IP phones

Key CDP Characteristics:

• Utilizes multicast MAC address 0100.0CCC.CCCC for message transmission
• Messages are processed locally and not forwarded to other devices
• Message interval: 60 seconds
• Holdtime: 180 seconds (neighbors are removed if no message is received within this period)

CDP Commands

# shows basic inoformation about CDP (timer, version)
R1#show cdp

# How many cdp packets, advertisements the device has sent / received
R1#show cdp traffic

# Gives some basic information about each interface
R1#show cdp interface

# View the neighbors
R1#show cdp neighbors

# lists each CDP neighbor with more detailed information
R1#show cdp neighbors detail

# displays the same info as above, but for the specified neighbor only
R1# show cdp entry name