What Is LLDP?

LLDP Frame Structure

An LLDP frame is an Ethernet frame encapsulated with an LLDP data unit (LLDPDU). The following figure shows the LLDP frame structure.

LLDP frame structure

An LLDP frame contains the following fields:

• DA: destination MAC address, a fixed multicast MAC address 0x0180-C200-000E
• SA: source MAC address, the MAC address of the sender
• Type: packet type, 0x88CC in LLDP frames
• LLDPDU: LLDP data unit, body of an LLDP frame
• FCS: frame check sequence

LLDPDU Structure

An LLDPDU contains local device information and is encapsulated in an LLDP frame. Each LLDPDU consists of several information elements known as Type-Length-Value (TLV) fields. A device encapsulates its local information into TLVs, assembles them into an LLDPDU, and then encapsulates the LLDPDU within an LLDP frame. The LLDPDU structure is illustrated in the following figure.

LLDPDU structure

As shown in the figure, an LLDPDU has four mandatory TLVs: Chassis ID TLV, Port ID TLV, Time to Live TLV, and End of LLDPDU TLV. Other TLVs are optional and may be included in an LLDPDU at the discretion of the device.

When LLDP is disabled on an interface or an interface is shut down, the interface sends a shutdown LLDPDU to its neighbors, with the Time to Live TLV set to 0 and no optional TLVs present.

TLV Structure

An LLDPDU is composed of multiple TLVs, each representing a distinct information element.

The following figure shows the structure of a TLV.

TLV structure

A TLV contains the following fields:

• TLV Type (7 bits): type of a TLV. Each TLV type has a unique value, such as 0 for End of LLDPDU and 1 for Chassis ID.
• TLV Length (9 bits): size of a TLV.
• TLV Value (0–511 bytes): The first bit indicates the TLV sub-type and the remaining bits carry the actual content.

TLV Type

An LLDPDU can encapsulate basic TLVs, IEEE 802.1-defined TLVs, IEEE 802.3-defined TLVs, and Media Endpoint Discovery (MED) TLVs. Basic TLVs are used for fundamental device management, while IEEE 802.1, IEEE 802.3, and MED TLVs provide enhanced management capabilities. Devices determine whether to include organizationally specific TLVs in LLDPDUs based on actual requirements.

• Basic TLVs

  The following table lists basic TLVs, four of which are mandatory in LLDP implementation and must be encapsulated in an LLDPDU.

  Table 1-1 Basic TLVs

  TLV	Description	Mandatory
  Chassis ID TLV	Bridge MAC address of the device sending an LLDPDU.	Yes
  Port ID TLV	Name of the port from which an LLDPDU is sent.	Yes
  Time To Live TLV	Time to live (TTL) of the local device information stored on the neighbor device.	Yes
  End of LLDPDU TLV	End of an LLDPDU.	Yes
  Port Description TLV	Character string that describes the port sending an LLDPDU.	No
  System Name TLV	Device name. NOTE: If the ip domain-name command has been configured to add a suffix to the device name, this TLV displays device name.suffix. For example, if the device name is MyDevice and suffix is area1, this TLV carries the value MyDevice.area1.	No
  System Description TLV	Character string that describes the system.	No
  System Capabilities TLV	Main functions of the system and the functions that have been enabled.	No
  Management Address TLV	Address used by the NMS to identify and manage the local device. Management addresses uniquely identify network devices, facilitating network topology layout and network management.	No

• TLVs defined by the IEEE 802.1 working group

  Table 1-2 IEEE 802.1-defined TLVs

  TLV	Description
  Port VLAN ID TLV	VLAN ID of a port.
  Port And Protocol VLAN ID TLV	Protocol VLAN ID of a port.
  VLAN Name TLV	Name of the VLAN on a port.
  Protocol Identity TLV	Protocol types that a port supports.

• TLVs defined by the IEEE 802.3 working group

  Table 1-3 IEEE 802.3-defined TLVs

  TLV	Description
  EEE TLV	Whether a port supports Energy Efficient Ethernet (EEE).
  Link Aggregation TLV	Whether a port supports link aggregation and has link aggregation enabled.
  MAC/PHY Configuration/Status TLV	Rate and duplex mode of a port, whether the port supports auto-negotiation, and whether auto-negotiation is enabled on the port.
  Maximum Frame Size TLV	Maximum frame length that a port supports. The value is the maximum transmission unit (MTU) of the port.
  Power Via MDI TLV	Power capabilities of a port, for example, whether a port supports PoE and whether a port supplies or demands power.

• MED TLVs

  MED TLVs are related to voice over IP (VoIP) applications and provide functions such as basic configuration, network policy configuration, address management, and directory management. These TLVs meet the requirements of voice device manufacturers for cost efficiency, easy deployment, and easy management. Use of these TLVs allows the deployment of voice devices on an Ethernet network. This brings great convenience for manufacturers, sellers, and users of voice devices.

  

  When a switch receives an LLDP packet containing any MED TLV from a neighbor on an interface, it responds by advertising all MED TLVs supported on that interface to the neighbor. If the neighbor device can recognize only some of these MED TLVs, LLDP negotiation fails. To avoid this, you can run the undo lldp tlv-enable med-tlv command to prevent the interface from advertising the MED TLVs not supported by its LLDP neighbor. For example, if an interface connects to a terminal that does not support the 802.3af standard and therefore cannot recognize Extended Power-via-MDI TLV, run the undo lldp tlv-enable med-tlv power-over-ethernet command on the interface to disable advertisement of that TLV.

  Table 1-4 MED TLVs

  TLV	Description
  LLDP-MED Capabilities TLV	Type of a device and types of MED TLVs that can be encapsulated in an LLDPDU.
  Inventory TLV	Manufacturer of the device.
  Location Identification TLV	Location of the local device.
  Network Policy TLV	VLAN ID, Layer 2 priority, and DSCP of a voice VLAN.
  Extended Power-via-MDI TLV	Power capability of the system.
  Hardware Revision TLV	Hardware version of a media endpoint (ME).
  Firmware Revision TLV	Firmware version of an ME.
  Software Revision TLV	Software version of an ME.
  Serial Number TLV	Serial number of an ME.
  Model Name TLV	Model name of an ME.
  Asset ID TLV	Asset identifier of an ME.

Mechanisms for Sending and Receiving LLDP Frames

Mechanism for sending LLDP frames

After LLDP is enabled on a device, it sends LLDP frames periodically to its neighbors and also when its configuration changes. To prevent LLDP frames from flooding the network due to frequent configuration changes, the device implements a delay before sending the next LLDP frame.

Mechanism for receiving LLDP frames

Upon receiving an LLDP frame from a neighbor, a device validates the frame and its TLVs. If the frame and TLVs are valid, the device stores the information about the neighbor and sets its aging time based on the TTL value in the LLDPDU. If the TTL is 0, the device immediately ages out the neighbor information.