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CX11x, CX31x, CX710 (Earlier Than V6.03), and CX91x Series Switch Modules V100R001C10 Configuration Guide 12

The documents describe the configuration of various services supported by the CX11x&CX31x&CX91x series switch modules The description covers configuration examples and function configurations.
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This section describes the implementation of LLDP.

LLDP Implementation

LLDP collects and sends local device information to remote devices, and the local device saves information received from remote devices to standard MIBs. Figure 14-20 shows how LLDP is implemented.

Figure 14-20 LLDP block diagram

LLDP is implemented as follows:

  1. The LLDP module uses an LLDP agent to interact with the Physical Topology MIB, Entity MIB, Interfaces MIB, and other MIBs to update the LLDP local system MIB and LLDP local organizationally defined extended MIB.
  2. The LLDP agent encapsulates local device information in LLDP frames and sends the LLDP frames to remote devices.
  3. After receiving LLDP frames from remote devices, the LLDP agent updates the LLDP remote system MIB and LLDP remote organizationally defined extended MIB.
  4. By exchanging LLDP frames with remote devices, the local device can obtain information about remote devices, including remote interfaces connected to the local device and MAC addresses of remote devices.

The LLDP local system MIB stores local device information, including the device ID, port ID, system name, system description, port description, and management address.

The LLDP remote system MIB stores neighbor information, including the device ID, port ID, system name, system description, port description, and management address of each neighbor.

An LLDP agent performs the following tasks:

  • Maintains the LLDP local system MIB and LLDP remote system MIB.
  • Obtains and sends LLDP local system MIB information to remote devices when the local device status changes. An LLDP agent also obtains and sends LLDP local system MIB information to remote devices at periodic intervals if the local device status does not change.
  • Identifies and processes received LLDP frames.
  • Sends LLDP traps to the NMS when information in the LLDP local system MIB or LLDP remote system MIB changes.

LLDP Frame Format

An LLDP frame is an Ethernet frame encapsulated with an LLDP data unit (LLDPDU). Figure 14-21 shows the LLDP frame format.

Figure 14-21 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.

An LLDPDU contains local device information and is encapsulated in an LLDP frame. Each LLDPDU consists of several information elements known as TLVs that each includes Type, Length, and Value 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 14-22 shows the LLDPDU structure.

Figure 14-22 LLDPDU structure

As shown in Figure 14-22, 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 14-23 shows the structure of a TLV.

Figure 14-23 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 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 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 14-18 Basic TLVs




    Chassis ID TLV

    Bridge MAC address of the device sending an LLDPDU.


    Port ID TLV

    Port from which an LLDPDU is sent.
    • If an LLDPDU does not contain any MED TLVs, the Port ID TLV identifies the port name.
    • If an LLDPDU contains a MED TLV, the Port ID TLV identifies the port MAC address. If the port has no MAC address, the Port ID TLV identifies the bridge MAC address.


    Time To Live TLV

    Time to live (TTL) of the local device information stored on the neighbor device.


    End of LLDPDU TLV

    End of an LLDPDU.


    Port Description TLV

    Character string that describes the port sending an LLDPDU.


    System Name TLV

    System name.


    System Description TLV

    Character string that describes the system.


    System Capabilities TLV

    Main functions of the system and the functions that have been enabled.


    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.


  • TLVs defined by the IEEE 802.1 working group

    Table 14-19 TLVs defined by the IEEE 802.1 working group



    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 14-20 TLVs defined by the IEEE 802.3 working group




    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.


    Data center bridging (DCB) information. Neighboring nodes on data center networks use the Data Center Bridging Exchange (DCBX) protocol to exchange and negotiate DCB information so that they have the same DCB information. This prevents packet loss on the data center network. DCBX encapsulates DCB information in DCBX TLVs and uses LLDP frames to exchange DCB information between neighboring nodes.

Transmission and Reception Mechanisms

LLDP frame transmission

  • After LLDP is enabled on a device, the device periodically sends LLDP frames to neighbors. When the local configuration changes, the device sends LLDP frames to notify neighbors of the changes. To reduce the number of LLDP frames sent when the local information changes frequently, the device waits for a period before sending the next LLDP frame.
  • The device starts fast transmission of LLDP frames in the following scenarios: when it receives an LLDP frame with device information not in its MIB (a new neighbor is discovered), when LLDP is enabled, or a when port transitions from Down to Up state. When fast transmission starts, the local device sends LLDP frames at 1-second intervals. After a specified number of LLDP frames have been sent at this interval, the device reverts the previous transmission interval.

LLDP frame reception

An LLDP-capable device checks the validity of received LLDP frames and the TLVs in those frames. When determining that an LLDP frame and its TLVs are valid, the local device saves neighbor information and sets the aging time of neighbor information on the local device to the TTL value carried in the received LLDPDU. If the TTL value carried in the received LLDPDU is 0, the neighbor information ages out immediately.

LLDP Networking

LLDP has the following networking modes:

  • Single-neighbor networking

    In this networking, interfaces between two switch moduless are directly connected, and each interface has only one neighbor. As shown in Figure 14-24, Switch ModuleA is directly connected to Switch ModuleB. Each interface on Switch ModuleA and Switch ModuleB has only one neighbor.

    Figure 14-24 Single-neighbor networking

  • Link aggregation networking

    Interfaces between switch moduless are directly connected and bundled into a link aggregation group. Each interface in a link aggregation group has only one neighbor. As shown in Figure 14-25, the interfaces connecting Switch ModuleA and Switch ModuleB are bundled into a link aggregation group, and each interface has only one neighbor.

    Figure 14-25 Link aggregation networking

Updated: 2019-08-09

Document ID: EDOC1000041694

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