Cisco Discovery Protocol Explained: Features, Commands, and Use Cases

 CDP (Cisco Discovery Protocol)

Cisco Discovery Protocol (CDP) is a proprietary Layer 2 network protocol developed by Cisco Systems. It is used to share information about directly connected Cisco devices, helping network administrators discover and manage network topology more efficiently.

Purpose of CDP

CDP allows Cisco devices to advertise their existence and capabilities to neighboring devices. It helps in:

• Network mapping
• Troubleshooting connectivity issues
• Verifying device configurations
• Identifying misconfigured or unauthorized devices

How CDP Works

• CDP operates at Layer 2 (Data Link Layer) of the OSI model.
• It sends periodic broadcast messages (CDP advertisements) to multicast MAC address 01:00:0C:CC:CC:CC.
• These messages contain information such as:
• Device ID (hostname)
• IP address
• Port ID
• Platform (hardware model)
• Capabilities (e.g., router, switch)
• Software version

CDP Packet Structure

Each CDP packet includes:

• Header: Protocol version and TTL (Time to Live)
• TLVs (Type-Length-Value): Encoded fields that carry device information

Common CDP Commands (Cisco CLI)

• show cdp neighbors: Displays directly connected Cisco devices
• show cdp neighbors detail: Provides detailed info, including IP addresses
• cdp enable: Enables CDP on an interface
• no cdp enable: Disables CDP on an interface
• cdp run: Enables CDP globally
• no cdp run: Disables CDP globally

Security Considerations

• CDP can expose sensitive network information if not properly secured.
• It should be disabled on interfaces connected to untrusted networks (e.g., internet-facing ports).
• Alternatives like LLDP (Link Layer Discovery Protocol) are preferred in multi-vendor environments.

Use Cases

• Network topology discovery
• Automated inventory management
• Troubleshooting and diagnostics
• VoIP deployments (e.g., auto-configuring IP phones)