Link Layer Discovery Protocol LLDP

Link Layer Discovery Protocol is a vendor-independent protocol that was developed by the Institute of Electrical and Electronics Engineers (IEEE) in the late 1990s. It was designed as an extension to the existing Ethernet standard, allowing network devices to communicate and share information about each other on a LAN.

LLDP operates at the link layer of the OSI model, allowing for end-to-end communication between multiple devices without relying on any particular protocol or setting up routing tables. It also enables the devices to detect and advertise their capabilities, including media type, duplex mode, and speed.

What is Link Layer Discovery Protocol (LLDP)?

Link Layer Discovery Protocol (LLDP) is a layer 2 protocol that enables network devices such as routers, switches, and wireless access points to communicate with each other. It allows the exchange of information between two or more network devices to discover the presence of other connected devices on a local area network (LAN). LLDP also enables devices to identify and describe each other, as well as keep track of the topology of its immediate network segment. 

Benefits of LLDP

Here are some of the main benefits of using LLDP within a network:

  • Reduces manual configuration and maintenance time: LLDP removes the need to manually configure and maintain IP addresses, switch ports, and other network elements. This can save time and effort as well as reduce operational costs.
  • Enables network devices to self-configure: As each device broadcasts its information, other connected devices can automatically detect it and begin communicating without any configuration or manual intervention. This makes the process of setting up and managing LANs easier and faster.
  • Improves network security: Since LLDP enables devices to identify each other, it helps prevent unauthorized access to a network as well as malicious activities such as man-in-the-middle attacks. LLDP also provides extra protection against DoS (Denial of Service) attacks.
  • Facilitates network troubleshooting: LLDP can help to diagnose and resolve network problems more efficiently, as it allows the IT staff to quickly identify which devices are connected to the LAN and how they are configured.

How Does LLDP Work?

The working of LLDP is quite simple, yet effective. It uses specially-formatted messages (called ‘TLVs’) that are broadcasted between devices on the same LAN segment. The TLVs contain information about each device, allowing them to identify and configure themselves without manual intervention.

Each device first sends out an advertisement packet containing its information. When another device receives the packet, it extracts the data and sends out its advertisement packet with its details. This process is repeated until all connected devices have received each other’s information.

LLDP also allows for additional communication between devices such as exchanging frames containing extra configuration parameters or troubleshooting diagnostics. This additional communication is done via a ‘link layer topology’ protocol, which provides extra information about the network’s structure and devices.

Common Features and Properties of LLDP

1. LLDP Packet Structure and Formatting

The Link Layer Discovery Protocol is based on a simple packet structure, consisting of four main sections: the Destination Address field, Source Address field, a Length field, and Type/Length/Value (TLV) fields.

The first two fields contain the MAC addresses of both devices that are involved in the communication. The Length field stores the size of the packet, while the TLV fields contain specific information about each device, such as its IP address, hostname, and capabilities.

2. Basic TLV Types Used in LLDP

TLVs are the main elements of an LLDP packet and they provide a wide range of information about a device, such as its manufacturer name, IP address, port type, duplex mode, and speed. There are several types of TLVs used in LLDP:

  • Chassis ID TLV: contains the chassis ID or system name of a device
  • Port ID TLV: stores the port number or interface number of the device
  • Time To Live (TTL) TLV: stores the time for which a packet is valid
  • Port Description TLV: contains information about the port, such as its type and capabilities
  • System Name TLV: contains the name of the system or network that is connected to this device
  • System Description TLV: contains detailed information about the system or network, such as its location and hardware configuration
  • System Capabilities TLV: provides information on the capabilities of a system, such as whether it has routing or bridging abilities

3. Protocol Operation and Timers

The Link Layer Discovery Protocol operates using two types of timers: the Hello timer and the Hold timer. The Hello timer is used to regulate how often a device sends out its advertisement packets, while the Hold timer determines the length of time that these packets are valid.

In general, devices will send out their TLV information every 30 seconds, while other connected devices can accept this data for up to 120 seconds. This helps to ensure that all devices on the network have received the most up-to-date information.

Use Cases of LLDP

1. Automating Device Discovery 

LLDP allows devices to be automatically discovered and identified on the network, removing the need for manual configuration. This makes setup faster and easier, as each device can send out its advertising packet containing its information. Other devices on the network will then receive this data and respond accordingly. This process also helps save time in managing networks with a large number of devices, as each device can be quickly identified and configured.

2. Configuration Deployment 

The Link Layer Discovery Protocol can also be used to deploy configurations across large networks of devices. This is done by exchanging TLV frames containing configuration information between two or more nodes on the network. This allows the same configuration to be sent out to all connected devices, reducing the manual effort required to configure each one.

3. Port Mapping, Inventory, and Troubleshooting 

LLDP also provides several benefits when it comes to port mapping, inventory, and troubleshooting. By exchanging information between two or more nodes on the network, administrators can build an up-to-date inventory of all connected devices, as well as their ports and capabilities. This allows for easy identification of problems, as well as quick troubleshooting when issues arise.

4. Interoperability with Other Networking Technologies 

Finally, LLDP can also be used to improve interoperability between different networking technologies. By exchanging TLV frames between devices, it can provide a layer of abstraction that allows for communication between different systems. This makes it easier to integrate new technologies into existing networks, as well as migrate from one networking technology to another without the need for manual configuration.


The Link Layer Discovery Protocol (LLDP) is an important network protocol used for automated device discovery, configuration deployment, port mapping, and inventory, as well as troubleshooting. It is also interoperable with other networking protocols, making it easier to integrate new technologies into existing networks. All of these features make LLDP an essential tool for managing large-scale networks.

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