Cisco Discovery Protocol (CDP)

Cisco Discovery Protocol (CDP) is a Layer 2, media-independent, and network-independent protocol that runs on Cisco devices to learn about directly connected devices.

This protocol facilitates the management of Cisco devices by discovering the directly connected devices, determining how they are configured and allowing systems, using different network-layer protocols, to learn about each other.

Restrictions for Using Cisco Discovery Protocol

Cisco Discovery Protocol is not supported on Frame Relay multipoint sub-interfaces.

Prerequisites for using CDP

Interfaces must support Subnetwork Access Protocol (SNAP) headers.

CDP_SNAP-1

  • CDP is a proprietary protocol so it works only on cisco devices.
  • CDP is a Layer 2 protocol.
  • Primary function of CDP is to learn about directly connected cisco devices.
  • CDP is enabled by default
  • Each device sends periodic messages to the well-known multicast MAC address ( 01:00:0C:CC:CC:CC )
  • Hellos are sent on every 60 seconds and expiration time is 180 seconds.

show_cdp-1

Type-Length-Value (TLV) fields are blocks of information embedded into CDP frames. TLV frame format allows for extending advertisement when needed.

Each TLV field is as below (output captured in Wireshark) :

  • CDP Version

CDP_Version_TLV-1

  • Device ID

CDP_Device_ID_TLV-1

  • Software Version

CDP_Software_Version_TLV-1

  • Platform

CDP_Platform_TLV-1

  • Addresses

CDP_Addresses_TLV-1

  • Port ID

CDP_Port_ID_TLV-1

  • Capabilities

CDP_Capabilities_TLV-1

  • IP Prefixes

CDP_IP_Prefix_TLV-1

  • Duplex

CDP_Duplex_TLV-1

To view these TLVs use following command.

show_cdp_ne_details

By default CDP is enabled on all interfaces to send and receive CDP information. As security concern, It might be risky to share this type of information. The Secure Cisco Discovery Protocol feature allows users to select the type, length, value (TLV) fields that are sent on a particular interface to filter information sent through Cisco Discovery Protocol. (will discuss in next blog)

To disable it, use “no cdp run” global configuration command & use “no cdp enable” interface specific command.

#DV

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s