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Configuration Guide - Ethernet Switching

S7700 and S9700 V200R011C10

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

Background

An Eth-Trunk in manual mode can increase the bandwidth. However, the manual mode can only detect member link disconnections, but 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, provide backup, and ensure high reliability of member links.

LACP uses a standard negotiation mechanism for a switching device so that the switching device can create and start the aggregated link based on its configuration. After the aggregated link is created, LACP maintains the link status. If an aggregated link's status changes, LACP adjusts or removes the 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. Because an interface on DeviceA is incorrectly connected to an interface on DeviceC, DeviceA may incorrectly send data destined for DeviceB to DeviceC. However, the Eth-Trunk in manual mode cannot detect this fault in a timely manner.

If LACP is enabled on DeviceA and DeviceB, the Eth-Trunk correctly selects active links to forward data after negotiation. Data sent by DeviceA can reach DeviceB.

Figure 3-3  Incorrect Eth-Trunk connection

Concepts

  • 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; otherwise, an LAG cannot be established. To keep active member interfaces consistent at both ends, set a higher priority for one end so that the other end selects active member interfaces based on the selection of the end with a higher priority. The smaller the LACP system priority value, the higher the LACP system priority.

  • LACP interface priority

    Interface LACP priorities are set to prioritize interfaces of an Eth-Trunk. Interfaces with higher priorities are selected as active interfaces. The smaller the LACP interface priority value, the higher the 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 mode is also called the M:N mode, where M refers to the number of active links and N refers to the number of backup links. This mode guarantees high reliability and allows traffic to be load balanced among M active links.

    As shown 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 load balanced among M active links and no data is transmitted over N backup links. Therefore, the actual bandwidth of the aggregated link is the sum of the M links' bandwidth, and the maximum bandwidth of the aggregated link is the sum of the M+N links' bandwidth.

    If one of M links fails, LACP selects a link from N backup links to replace the faulty link. The actual bandwidth of the aggregated link is still the sum of M links' bandwidth, but the maximum bandwidth of the aggregated link is the sum of the (M+N-1) links' bandwidth.

    Figure 3-4  Networking of M:N backup

    M:N backup is mainly applied in situations where the bandwidth of M links must be assured and a fault tolerance mechanism is in place. If an active link fails, the system selects the backup link with the highest priority as the active link.

    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 information with that saved on itself, 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 meaning of each field is explained as follows:
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.
  • An Eth-Trunk in LACP mode is set up as follows:

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

      As shown 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. Then the member interfaces are enabled with LACP, and devices at both ends can send LACPDUs to each other.

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

      As shown in Figure 3-7, devices at both ends receive LACPDUs from each other. For example, when DeviceB receives LACPDUs from DeviceA, DeviceB checks and records information about DeviceA and compares system priorities. If the system priority of DeviceA is higher than that of DeviceB, DeviceA acts as the Actor. If DeviceA and DeviceB 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 interface priority of the Actor. 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 function as active interfaces.

    As shown in Figure 3-8, Port 1, Port 2, and Port 3 are member interfaces of an Eth-Trunk; DeviceA acts as the Actor; the upper threshold for the number of active interfaces is 2; LACP priorities of Port 1, Port 2, and Port 3 are 10, 20, and 30 respectively. When LACP negotiation is complete, Port 1 and Port 2 are selected as 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 situations:

    • Port 1 becomes faulty, and then recovers. When Port 1 fails, Port 3 replaces Port 1 to transmit services. After Port 1 recovers, if LACP preemption is not enabled on the Eth-Trunk, Port 1 still retains in backup state. If LACP preemption is enabled on the Eth-Trunk, Port 1 and Port 3 become the active interface and backup interface respectively.
    • If LACP preemption is enabled and Port 3 needs to replace Port 1 or Port 2 to become the active interface, set the highest LACP priority value for Port 3. When LACP preemption is not enabled, the system does not re-select the active interface even if the priority of a backup interface is higher than that of the active interface.
  • LACP preemption delay

    After LACP preemption occurs, a backup link waits for a given period of time and then switches to the active status. 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.

    As shown in Figure 3-8, Port 1 becomes inactive due to a link fault. Then the link of Port 1 recovers. If LACP preemption is enabled and the LACP preemption delay is set, Port 1 switches to be active after the LACP preemption delay.

  • 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, perform the following operations:

    1. Shut down the faulty link.

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

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

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Updated: 2019-10-18

Document ID: EDOC1000178310

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