LLDP – Link Layer Discovery Protocol

LLDP – Link Layer Discovery Protocol

- in Enterprise Infrastructure

LLDP is similar to the CDP protocol but is based on the IEEE 802.1ab standard. The great advantage of LLDP is that it works on multi-vendor networks.

Information is exchanged via multiple TLVs (Type-Length-Value). One TLV is sent for the system name, one for its management address, one for the port description, and so on. In the end the complete LLDP announcement becomes a string of multiple TLVs.

Each frame sent contains one LLDP Data Unit (LLDPDU).

By default LLDP is not enabled on cisco switches. To enable it, we use the following command:

Switch(config)# lldp run           Enables LLDP globally on device.
Switch(config)# no lldp run        Disables LLDP globally on device.

LLDP supports three different modes of operation that are configured at the interface level:

1. Transmit only It only sends LLDP packets and discards those it receives.
2. Receive only – It only processes the LLDP packets it receives; it does not send LLDP packets to its neighbor.
3. Both transmit and receive – Saves the information of received packets and sends LLDP announcements to neighbors.

By default, it sends and receives LLDP packets, if you want to modify the operation modes on the interfaces, use the following commands:

(config-if)# lldp {receive | transmit}
(config-if)# no lldp {receive | transmit}

LLDP Timers:

  • Hold time: -It is the duration that the switch will keep the neighbor’s information; by default it is 120 seconds, if after 120 seconds the switch does not receive an LLDP packet, the information is deleted.
  • LLDP Packet Frequency timer: Time interval in which the device sends LLDP packets to its neighbors; default is 30 seconds.
  • Reinit time: Delay time in seconds for LLDP to initialize on any interface. The default value is 2 seconds.
(config)# lldp holdtime <seconds>   
(config)# lldp timer <seconds>    
(config)# lldp reinit <seconds>

Once LLDP is enabled, advertisements are sent and received on each switch interface.

LLDP verification commands are shown below:

clear lldp counters
clear lldp table
show lldp
show lldp entry <* | entry-name>
show lldp interface [interface-id]
show lldp neighbors [interface-id] [detail]
show lldp traffic

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