Neighbour Discovery Protocols - Part 2 ( LLDP ) - :
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LLDP (Link Layer Discovery Protocol)

LLDP (Link Layer Discovery Protocol) is a Standard Neighbour Discovery Protocol that is used by all vendors. Like all other Neighbour Discovery Protocols, LLDP works on Layer 2 (Data-link Layer). By using LLDP, directly connected neighbours are discovered.

To discover a neighbour device, LLDP must be enabled both on the discovering device and the discovered device.

With LLDP, ethernet devices advertise their identification, configuration etc. to the directly connected LLDP enabled devices.

As we mentioned before, there are many neighbour discovery protocol and they are vendor specific. To build a one common protocol, LLDP (Link Layer Discovery Protocol) is developed and it is used on all vendors’ devices. LLDP is independent from the brand of the device used.

Hello Timer is 30 seconds for LLDP and Dead Timer is 120 seconds.

LLDP has an extension version that provide end devices’s discovery. With this extention, LLDP discovers the endpoint PCs, IP Phone. This extended version of LLDP is LLDP-MED (Media Endpoint Discovery). We will talk about LLDP-MED, in the following articles. Here, in this article, our main focus is LLDP.

How LLDP Works?

As we have mentioned above, LLDP must be enabled on the devices firstly. After that, LLDP enabled devices send LLDP advertisements each other and the device information are stored in the MIB databases on the devices. Any Network Management Software gets this data by SNMP (Simple Network Management Protocol). And with this data, it builds the network map.

How LLDP Works, LLDP Mechanism

How LLDP Works, LLDP Mechanism

When the device receive a new information about the LLDP neightbour device, it stores this neighbour information in the MIB (Management Information Base). There are two MIB. LLDP Local System MIB and LLDP Remote System MIB. LLDP Local System MIB stores the information about the local device, the device’s own information. LLDP Remote System MIB stores the information gathered from the LLDP neightbour devices.

When the neighbour information come to the device, it stores it in the MIB. And whenever a new information comes, it updates the MIB. If no information come in a certain time, it deletes the neighbour’s information from the MIB (Management Information Base).

The devices that is using LLDP, has a LLDP Agent Software. This LLDP Agent is responsible for all the LLDP actions.

LLDP is a One-way protocol, it send in one direction and do not wait for acknowledgement. In LLDP the advertisements are sent with LLDPDUs (LLDP Data Units).

When you enable LLDP on a port, it can transmit and receive. But you can seperately configure them also.

There are three modes in LLDP. These modes are :

• Transmit only
• Receive only
• Both

Ethernet Frame and LLDP TLVs (Type Lenght Value)

LLDP information is sent in the ethernet frame form. LLDP has no specific Header.

AS you know, in the ethernet frame, there are source and destination mac address fields. After this fields, there is a Ether Type field. In this field, 0x88CC value is used for LLDP.

After using 0x88CC Ether Type value, LLDP Data Unit (LLDPDU) is added to the frame. LLDP Data Unit (LLDPDU) is consist of LLDP TLVs (Type Lenght Value). A TLV (Type Lenght Value) advertises a single type of information. This information can be any information about the neighbour device.

LLDP Data Unit, TLVs

LLDP Data Unit, TLVs

There are two types of TLVs :

• Mandatory TLVs
• Optional TLVs

Mandatory TLVs
must be in the LLDP message. Optional TLVs are optional in the LLDP messages.

By default there are four TLVs in a LLDPDU (Mandatory TLVs).
These Mandatory TLVs are :

• Chassis ID (Switch MAC)
• Port ID
• End of LLDPDU ( means no more TLVs)

You can add five more additional Optional TLVs to a LLDPDU.
These Optional TLVs are:

• System Name
• System Description
• System Capabilities
• Port Description
• Management Address (Local LLDP Agent Address)



Above, you can see the fields in the LLDP message. In this example header, you can see the mandatory TLVs and the Optional TLVs together. The information about the devices are stored in these fields.

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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