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S12700 V200R010C00 Configuration Guide - Ethernet Switching

This document describes the configuration of Ethernet services, including configuring link aggregation, VLANs, Voice VLAN, VLAN mapping, QinQ, GVRP, MAC table, STP/RSTP/MSTP, SEP, and so on.
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Link Aggregation in LACP Mode

Link Aggregation in LACP Mode


While an Eth-Trunk in manual mode can increase bandwidth, it can only detect member link disconnections. It cannot detect other faults such as link layer faults and incorrect link connections.

The Link Aggregation Control Protocol (LACP) can improve fault tolerance of the Eth-Trunk, implement backup, and ensure high reliability of member links.

LACP provides a standard negotiation mechanism for a switching device so that the switching device can be configured to automatically form and start aggregated links. After an aggregated link is formed, LACP is responsible for maintaining the link. If link aggregation conditions or requirements change, LACP can adjust or remove the aggregated link.

For example, in Figure 3-3, four interfaces on DeviceA are bundled into an Eth-Trunk and the Eth-Trunk is connected to the corresponding interfaces on DeviceB. One of the interfaces on DeviceA is incorrectly connected to an interface on DeviceC so DeviceA may incorrectly send data destined for DeviceB to DeviceC. An Eth-Trunk in manual mode cannot quickly detect this fault.

If LACP is enabled on DeviceA and DeviceB, the Eth-Trunk correctly selects active links to forward data after LACP negotiations. This ensures that data reaches the correct destination.

Figure 3-3  Incorrect Eth-Trunk connection


  • LACP system priority

    LACP system priorities are set on devices at both ends of an Eth-Trunk. In LACP mode, active member interfaces selected by both devices must be consistent with each other; otherwise, an LAG cannot be established. To ensure consistency between active member interfaces at both ends, set a higher priority for one device. The remote device will select active member interfaces based on the priority. A smaller LACP system priority value indicates a higher LACP system priority.

  • LACP interface priority

    Interface LACP priorities are used to prioritize interfaces of an Eth-Trunk. Interfaces with higher priorities are selected as active interfaces. A smaller LACP interface priority value indicates a higher LACP interface priority.

  • M:N backup of member interfaces

    In LACP mode, LACP is used to negotiate parameters to determine active links in an LAG. This is also called the M:N mode, where M is the number of active links and N is the number of backup links. This mode guarantees high reliability and allows traffic to be load balanced among the active links.

    In Figure 3-4, M+N links with the same attributes (in the same LAG) are set up between two devices. When data is transmitted over the aggregated link, traffic is only load balanced between the active links; no data is transmitted over the backup links. Therefore, the actual bandwidth of the aggregated link is the sum of the active links' bandwidth, and the maximum bandwidth of the aggregated link is the total bandwidth between the active and backup links.

    If one of active links fails, LACP selects a link from the backup links to replace the faulty link. The actual bandwidth remains the same, but the maximum bandwidth of the aggregated link is reduced accordingly.

    Figure 3-4  Networking of M:N backup

    M:N backup is mainly applied to ensure a consistent bandwidth between two devices. If no available backup link is found and the number of active links is smaller than the lower threshold for the number of active interfaces, the system shuts down the LAG.

Implementation of Link Aggregation in LACP Mode

LACP, as specified in IEEE 802.3ad, implements dynamic link aggregation and de-aggregation, allowing both ends to exchange Link Aggregation Control Protocol Data Units (LACPDUs).

After member interfaces are added to an Eth-Trunk in LACP mode, each end sends LACPDUs to inform its remote end of its system priority, MAC address, member interface priorities, interface numbers, and keys. The remote end then compares this to its own information and selects which interfaces to be aggregated. The two ends perform LACP negotiation to select active interfaces and links.

Figure 3-5 shows the format of an LACPDU.

Figure 3-5  Fields in an LACPDU
The following table describes the meaning of each field.
Item Description
Actor_Port/Partner_Port Interface of the Actor or Partner.
Actor_State/Partner_State Status of the Actor or Partner.
Actor_System_Priority/Partner_System_Priority System priority of the Actor or Partner.
Actor_System/Partner_System System ID of the Actor or Partner.
Actor_Key/Partner_Key Operational key of the Actor or Partner.
Actor_Port_Priority/Partner_Port_Priority Interface priority of the Actor or Partner.
The device with the higher system priority becomes the Actor. If the two devices have the same system priority, the device with a smaller MAC address functions as the Actor.
  • An Eth-Trunk in LACP mode is set up as follows:

    1. Devices at both ends send LACPDUs to each other.

      In Figure 3-6, you need to create an Eth-Trunk in LACP mode on DeviceA and DeviceB and add member interfaces to the Eth-Trunk. LACP then is enabled on the member interfaces, and devices at both ends send LACPDUs to each other.

      Figure 3-6  LACPDUs sent in LACP mode
    2. Devices at both ends determine the Actor and active links.

      In Figure 3-7, devices at both ends receive LACPDUs from each other. When each device receives LACPDUs from the other device, they check and record information about that device and compare system priorities. The device with the higher system priority becomes the Actor. If the two devices have the same system priority, the device with a smaller MAC address functions as the Actor.

      After the Actor is selected, devices at both ends select active interfaces based on the priorities of the Actor's interfaces. If priorities of interfaces on the Actor are the same, interfaces with smaller interface numbers are selected as active interfaces. An Eth-Trunk is established when devices at both ends select consistent interfaces. Active links load balance data.

      Figure 3-7  Selecting the Actor in LACP mode
  • LACP preemption

    When LACP preemption is enabled, interfaces with higher priorities in an LAG will always be the active interfaces as long as they are available.

    In Figure 3-8, Port 1 and Port 2 are active interfaces because their LACP priorities are higher, and Port 3 is used as the backup interface.

    Figure 3-8  LACP preemption

    LACP preemption is used in the following scenarios:

    • Port 1 becomes faulty, causing Port 3 to replace Port 1 to transmit services. After Port 1 recovers, if LACP preemption is not enabled on the Eth-Trunk, Port 1 remains in the backup state. If LACP preemption is enabled on the Eth-Trunk, Port 1 will replace Port 3 once it is restored.
    • With LACP preemption enabled, setting a higher LACP priority value for Port 3 will allow it to replace Port 1 or Port 2 as an active interface. If LACP preemption is not enabled, the system does not re-select active interfaces even if the priority of a backup interface is set higher than that of an active interface.
  • LACP preemption delay

    If a backup link is switched to an active link through LACP preemption, it will wait for a set period of time before switching. This period is called LACP preemption delay. The LACP preemption delay is used to prevent unstable data transmission over an Eth-Trunk link caused by frequent status changes of member links.

  • Switchover between active and inactive links

    In LACP mode, a link switchover in an LAG is triggered if a device at one end detects one of the following events:

    • An active link goes Down.

    • Ethernet OAM detects a link fault.

    • LACP detects a link fault.

    • An active interface becomes unavailable.

    • When LACP preemption is enabled, a backup interface's priority is changed to be higher than that of the current active interface.

    When any of the preceding events occurs, the following actions are performed:

    1. Shut down any faulty link.

    2. Select the backup link with the highest priority among the backup links to replace the faulty active link.

    3. The highest priority backup link becomes the active link and begins forwarding data.

Updated: 2019-08-21

Document ID: EDOC1000142081

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