LLDP Frame Format

An LLDP frame is an Ethernet frame encapsulated with an LLDP data unit (LLDPDU). Figure 3-2 shows the LLDP frame format.

Figure 3-2 LLDP frame format

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

An LLDPDU contains local device information and is encapsulated in an LLDP frame. Each LLDPDU consists of several information elements known as Type, Length, and Value (TLV) fields. The local device encapsulates its local information in TLVs, constructs an LLDPDU with several TLVs, and encapsulates the LLDPDU in the data field of an LLDP frame. Figure 3-3 shows the LLDPDU structure.

Figure 3-3 LLDPDU structure

As shown in Figure 3-3, 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 a device can determine whether to encapsulate them in an LLDPDU.

When LLDP is disabled on an interface or an interface is shut down, the interface sends a shutdown LLDPDU to the neighbors. In the shutdown LLDPDU, the value of the Time to Live TLV is 0. A shutdown LLDPDU contains no optional TLVs.

TLV Structure

An LLDPDU is formed by TLVs, and each TLV is an information element.

Figure 3-4 shows the structure of a TLV.

Figure 3-4 TLV structure

A TLV contains the following fields:

• TLV Type (7 bits): type of a TLV. Each TLV type has a unique value. For example, the value of End of LLDPDU TLV is 0, and the value of Chassis ID TLV is 1.
• TLV Length (9 bits): size of a TLV.
• TLV Value (0-511 bytes): The first bit indicates the sub-type of a TLV, and the other bits are the TLV content.

TLV Type

LLDPDUs can encapsulate basic TLVs, TLVs defined by the IEEE 802.1 working groups, TLVs defined by IEEE 802.3 working groups, and Media Endpoint Discovery (MED) TLVs. Basic TLVs are used for basic device management. The TLVs defined by the IEEE 802.1 and IEEE 802.3 working groups, and MED TLVs defined by other organizations are used for enhanced device management functions. A device determines whether to encapsulate organizationally specific TLVs.

• Basic TLVs

  Four basic TLVs are mandatory in LLDP implementation and must be encapsulated in an LLDPDU.

  Table 3-1 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.	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 IP addresses uniquely identify network devices, facilitating layout of the network topology and network management.	No

• TLVs defined by the IEEE 802.1 working group

  Table 3-2 TLVs defined by the IEEE 802.1 working group

  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 3-3 TLVs defined by the IEEE 802.3 working group

  TLV	Description
  EEE TLV	Specifies whether a port supports Energy Efficient Ethernet (EEE).
  Link Aggregation TLV	Specifies 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 Ethernet network. This brings great convenience for manufacturers, sellers, and users of voice devices.

  When the switch detects that the LLDP packet sent by an LLDP neighbor on an interface contains any type of MED TLV, the switch advertises all MED TLVs that can be advertised on the interface to the LLDP neighbor. However, the LLDP neighbor may support only parts of MED TLVs advertised by the switch, leading to an LLDP negotiation failure. You can run the undo lldp tlv-enable med-tlv command to enable the interface not to advertise the MED TLV that is not supported by the LLDP neighbor. For example, if a terminal does not support the 802.3af standard, that is, Extended Power-via-MDI TLV cannot be identified, run the undo lldp tlv-enable med-tlv power-over-ethernet command on the interface connected to the terminal to enable the interface not to advertise Extended Power-via-MDI TLV.

  Table 3-4 LLDP-MED TLVs

  TLV	Description
  LLDP-MED Capabilities TLV	Type of a device and types of LLDP-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 device.
  Software Revision TLV	Software version of an ME device.
  Serial Number TLV	Serial number of an ME device.
  Model Name TLV	Model name of an ME device.
  Asset ID TLV	Asset identifier of an ME device.