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Link layer discovery protocol explanation

With the development of network technology, there are more and more types of devices accessing the network, and the configuration is more and more complicated. Devices from different device vendors also tend to add their own unique features, which leads to many different features in a network. Devices from different vendors, in order to facilitate the management of such networks, need to enable devices from different vendors to discover and interact with each other’s systems and configuration information in the network.


LLDP (Link Layer Discovery Protocol) is generated in this context, which provides a standard link layer discovery method. The information about the main device, the management address, the device identifier, and the interface identifier of the local device can be organized into different TLV (Type/Length/Value)  and encapsulated in the LLDPDU (Link Layer Discovery Protocol Data Unit)  to be directly connected to the neighbors. After receiving the information, the neighbor saves it in the form of a standard MIB (Management Information Base) for the network management system to query and judge the communication status of the link.


These MIB information can be used to discover the physical topology of the device and manage configuration information. It should be noted that LLDP is only designed for information advertisement. It is used to advertise information of one device and obtain information of other devices to get related MIB information. It is not a configuration and control protocol. It can’t configure the remote device through the protocol. It only provides information about the network topology and management configuration. These information can be used for management, configuration purposes, but how to use it depend on the users.


In May 2005, this agreement has been recognized as the IEEE802.1AB-2005 standard. It replaces vendor-private protocols such as Cisco’s Cisco Discovery Protocol, Extreme Networks’ EDP (Extreme Discovery Protocol), Enterasys Networks’ CDP (Cabletron Discovery Protocol), and Nortel Networks’ NDP ( Nortel Discovery Protocol).


Simply put, LLDP is a proximity discovery protocol. It defines a standard way for Ethernet network devices, such as switches, routers, and WLAN access points, to advertise their presence to other nodes in the network and to store discovery information for neighboring devices. For example, detailed information such as device configuration and device identification can be announced with this protocol.


LLDP is an information discovery and advertisement protocol. The LLDP entity mainly maintains two MIB libraries, a local system MIB and a remote system MIB. It can also be seen from its name that one is used to maintain local related device MIB information, and one is used to maintain remote device MIB information.

LLDP initializes and maintains the local system MIB by interacting with several MIB libraries on the right side in the figure above, and advertises local related information. At the same time, when it receives information from other devices, it is updated to the remote system MIB. In this way, a device can advertise its own information and obtain information about other devices in the network, and finally obtain two MIB libraries that reflect the network topology and other configuration information. These two libraries can be used by their users to perform various functions.

LLDP packet:

A packet encapsulated with an LLDPDU is called an LLDP packet. The encapsulation format is two types: Ethernet II and SNAP (Subnetwork Access Protocol).

LLDP packets encapsulated in Ethernet II format:

  • Destination MAC address, which is a fixed multicast MAC address 0x0180-C200-000E.
  • Source MAC address, which is the port MAC address or device bridge MAC address (if there is a port address, the port MAC address is used, otherwise the device bridge MAC address is used).
  • Type, or Packet type, which is 0x88CC.
  • Data, mainly is LLDPDU.
  • FCS, or Frame check sequence.
  • Destination MAC address, which is a fixed multicast MAC address 0x0180-C200-000E.
  • Source MAC address, which is the port MAC address or device bridge MAC address (if there is a port address, the port MAC address is used, otherwise the device bridge MAC address is used).
  • Type, which is Packet type, which is 0xAAAA-0300-0000-88CC.
  • Data or LLDPDU.
  • FCS (Frame check sequence).

Sending mechanism

When a port works in TxRx or Tx mode, the device periodically sends LLDP packets to neighbors. If the local configuration of the device changes, the LLDP packet is sent immediately to notify neighbors of the change of local information. However, in order to prevent large-scale transmission of LLDP packets due to frequent changes of local information, each LLDP packet needs to be delayed for a period of time before continuing to send the next packet.

When the working mode of the device is switched from Disable/Rx to TxRx/Tx, or a new neighbor device is discovered (that is, a new LLDP packet is received and the device that sent the packet is not saved locally).The fast-sending mechanism will be enabled on the device. The LLDP packet transmission period is shortened to 1 second. The specified number of LLDP packets are sent to the normal transmission interval.

Receiving mechanism

When the port works in TxRx or Rx mode, the device checks the validity of the received LLDP packets and the TLVs they carry. After checking, the device saves the neighbor information to the local device and uses TTL (Time To Live). The value of the TTL in the TLV is used to set the aging time of the neighbor information on the local device. If the value is zero, the neighbor information is aged out.

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