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Network Discovery Protocols : CDP ( Cisco Discovery Protocol) and LLDP (Link Layer Discovery Protocol)

June 24, 2020
Today I am going to talk about the CDP and LLDP protocols. As you guys already knew of CDP, I think most of you are using this command in your daily use to find the connected devices in Cisco devices environment while LLDP is same as CDP but it is a standard protocol for all other Vendors.

So If I am talking about the CDP, CDP Stands for Cisco Discovery Protocol and LLDP stands for Link Layer Discovery Protocol. The role and the working is same for both of these protocols.

CDP is a Cisco proprietary protocol and can be used only on Cisco devices While  LLDP is the standard protocol and can be used for interworking between different network vendors. We need to know only CDP with regards to discovery protocols. But it is always good to go beyond and above what is required in the exam topics. 


Fig 1.1- CDP and LLDP Protocols


You might find yourself in the situation where you have to monitor and configure a multi-vendor network. LLDP knowledge will come in handy in that situation.

Some of the important facts about CDP Protocols are given below:-
  • CDP is used to discover the directly related neighboring gadgets, the IP addresses, and other records, consisting of platform type and the software of those devices.
  • CDP is a Layer 2 protocol this is runs on all media that support SNAP (subnet get admission to protocol). this means that it's far supported on Ethernet, body relay, and ATM mediums.
  • CDP version 2 is the modern day release of the protocol. compared to model 1, it gives upgrades concerning the reporting of numerous configuration mistakes of the two neighbor devices, which includes native VLAN mismatch and duplex mismatch.
  • Each CDP-enabled device sends periodic advertisements to a multicast vacation spot cope with: 01-00-0c-cc-cc-cc. due to the fact this is a multicast cope with, any CDP-enabled device will method the body. among different important statistics in each advertisement, the tool sends a maintain time. 

This c programming language instructs the receiving tool how long to preserve the CDP facts before affirming that the neighbor is useless. whenever a new commercial is obtained, the hold time is reset to the value found in the periodic commercial.

The strength of CDP is represented by means of the statistics contained within the CDP commercials as TLV (kind-length-cost). those TLVs describes what facts approximately the sending tool is dispatched over a CDP-enabled interface.

By default, CDP version 2 is enabled on all Cisco devices, on all interfaces. As expected, there are some default values regarding the advertisement timer and hold time timer. These are 60 seconds and 180 seconds. This means that if a device doesn’t hear anything for 180 seconds from the CDP neighbor, then that device is removed from CDP table as a CDP neighbor.

Now that we have some information about CDP and how is it working, it’s time to go further with the configuration and the monitoring.

The most basic command to check the advertisement interval, the hold time interval and the version of CDP is by using “show cdp”:


To list all the CDP neighbors, altogether with some basic information about them, you can the “show cdp neighbors’ command:


Check LLDP neighbors as below