Neighbour Discovery Protocols - Part 3 ( CDP ) - :
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Cisco Discovery Protocol (CDP)

CDP (Cisco Discovery Protocol) is a Cisco specific neighbour discovery protocol works on Layer 2. CDP (Cisco Discovery Protocol) is also licensed to run on some network devices of other vendors.

By using CDP, Cisco devices discover their directly connected Cisco neighbours. But to do this, CDP must be enable on both devices. By default CDP is enabled on Cisco devices.

Network Neighbour DiscoveryNeighbour Discovery Protocols

Cisco Discovery Protocol

CDP provides many important information about the neighbour device. Device IDs, ip addreses, capacities etc. are provided to the requested neighbour by using CDP (Cisco Discovery Protocol).

There are two version of CDP. These are, Version 1 and Version 2. The information that is collect is increased by the new version, version 2. And CDP version 2 is the default one.

If the device runs CDP version 1, then it ignores the CDP version 2 frames. But if the device runs CDP version 2, then it so not ignore any packet. It behaves like version 1 to version 1, like version 2 to version 2 devices.

CDP is teh protocol that can run on the SNAP ( Subnet Access Protocol ) supported medias. Ethernet, Token Ring, Frame Relay, ATM, HDL and PPP support SNAP, so CDP can run on these medias.

How CDP Works ?

First of all, to use CDP, CDP must be enabled on both end devices. Each CDP enabled device send periodic messages (advertisements) to the multicast adress ” 01:00:0C:CC:CC:CC ” and makes its information available to the nodes which are listenning to it. This periodic advertisements sent every 60 seconds ( Hello Time ).

The informations collected from the devices are stored in a MIB (Management Information Base). This is called Cisco CDP MIB .

Any CDP enabled device can use this information and until the end of the Hold Time, the device information is kept in the MIB. By default the Hold Time is 180 seconds, and the Hello Time that multicast advertisements are sent is 60 seconds.

CDP supports some attributes for device discovery. These attributes are called TLVs. Tyepe, Length and Value.

The CDP frame contains TLV (Type Lenght Value) fields. These TLVs are contains special information of the connected device.

CDP Header and TLVs

Unlike LLDP, CDP has Header. CDP frames consist of a Header and the following TLVs. Header is consist of Version, TTL and Checksum fields. After Header, TLVs (Type Lenght Value) are added to the frame.

Cisco Discovery Protocol Header and TLVs, CDP TLVs

Cisco Discovery Protocol Header and TLVs

Some of the TLVs of CDP is below:

• Device ID
• Local Interface
• Port ID
• Capabilities
• Version
• Platform
• IP Network Prefix
• Hold time

Cisco Discovery Protocol TLVs, CDP TLVs

Cisco Discovery Protocol TLVs, CDP TLVs

Neighbour Discovery Protocols – Part 1
Neighbour Discovery Protocols – Part 2 ( LLDP )
Neighbour Discovery Protocols – Part 3 ( CDP )
Neighbour Discovery Protocols – Part 4 ( CDP Configuration Example )
Neighbour Discovery Protocols – Part 5 ( LLDP-MED )
Neighbour Discovery Protocols – Part 6 ( LLDP Cisco Configuration)

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About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

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