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Configuration Guide - Ethernet Switching

S1720, S2700, S5700, and S6720 V200R012(C00 and C20)

This document describes the configuration of Ethernet services, including configuring MAC address table, link aggregation, VLANs, VLAN aggregation, MUX VLAN, VLAN termination, Voice VLAN, VLAN mapping, QinQ, GVRP, VCMP, STP/RSTP/MSTP, VBST, SEP, RRPP, ERPS, LBDT, and Layer 2 protocol transparent transmission.
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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. In manual mode, however, only member link disconnections can be detected. Other faults, such as link layer faults and incorrect link connections, cannot be detected.

LACP can be used to detect more fault types, improving fault tolerance of the Eth-Trunk, providing backup, and ensuring high reliability of member links.

LACP provides a standard negotiation mechanism that a switching device can use to create and start the aggregated link based on its configuration. LACP maintains the link status after the aggregated link is created and adjusts or removes the link if an aggregated link's status changes.

For example, in Figure 3-3, four interfaces on DeviceA should be connected to the corresponding interfaces on DeviceB, and these interfaces are all bundled into an Eth-Trunk. However, one interface on DeviceA has been connected to an interface on DeviceC. As a result, DeviceA may send data destined for DeviceB to DeviceC. If link aggregation in manual mode is used, this fault would go undetected.

In this situation, if LACP is enabled on DeviceA and DeviceB, the Eth-Trunk only selects active links (links connected to DeviceB) to forward data after negotiation. Data sent by DeviceA destined for DeviceB only reaches DeviceB.

Figure 3-3  Incorrect Eth-Trunk connection

Concepts

  • LACP system priority

    LACP system priorities determine the sequence in which devices at two ends of an Eth-Trunk select active interfaces to join a LAG. In order for a LAG to be established, both devices must select the same interfaces as active interfaces. To achieve this, one device (with a higher priority) is responsible for selecting the active interfaces. The other device (with a lower priority) then selects the same interfaces as active interfaces. In priority comparisons, numerically lower values have higher priority.

  • LACP interface priority

    Interface LACP priorities affect which interfaces of an Eth-Trunk are selected as active interfaces. The smaller the LACP interface priority value, the higher the LACP interface priority. The interfaces with the highest LACP interface priority become active interfaces.

  • M:N backup of member interfaces

    In LACP mode, LACP is used to negotiate parameters to determine active links in a 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 the 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 the M links must be assured and a fault tolerance mechanism is required. 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 devices at both ends of the link to exchange Link Aggregation Control Protocol Data Units (LACPDUs).

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

Figure 3-5 shows the fields in an LACPDU.

Figure 3-5  Fields in an LACPDU
The following table provides a description of the fields relevant to the Actor and Partner:
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 exchange LACPDUs.

      As shown in Figure 3-6, create an Eth-Trunk in LACP mode on DeviceA and DeviceB and add member interfaces to the Eth-Trunk. Then enable LACP on the member interfaces. Both devices can then exchange LACPDUs.

      Figure 3-6  Exchange of LACPDUs
    2. Devices at both ends determine the Actor and active links.

      As shown in Figure 3-7, 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 becomes the Actor. If DeviceA and DeviceB have the same system priority, the device with a smaller MAC address becomes the Actor.

      After the Actor is selected, devices at both ends select active interfaces based on the interface priorities 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 the same interfaces as active interfaces. After Eth-Trunk is established, active links load balance data.

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

    When LACP preemption is enabled, interfaces with higher priorities in a LAG always 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 has the following effects on selection of the active interfaces:

    • LACP preemption allows the original active interface to be re-selected after recovering from a fault. For example, when Port 1 fails, Port 3 replaces Port 1 as the active interface. If LACP preemption is not enabled on the Eth-Trunk, Port 1 remains in the backup state after it recovers. If LACP preemption is enabled on the Eth-Trunk, Port 1 replaces Port 3 to become the active interface again.
    • LACP preemption allows active interfaces to be re-selected when the LACP interface priority changes. For example, the LACP interface priority of Port 3 is changed to 5. If LACP preemption is enabled, Port 3 will replace Port 2 as an active interface.
  • LACP preemption delay

    The LACP preemption delay is the time a backup link waits before becoming the active link in scenarios where LACP preemption occurs. 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 becomes active again after the LACP preemption delay.

  • Switchover between active and inactive links

    In LACP mode, a link switchover in a 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 becomes the active interface when its priority is changed to be higher than that of the current active interface.

    When any of the preceding events occurs, LACP takes effect in the following sequence:

    1. Shuts down the faulty link.

    2. Selects 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: 2018-12-24

Document ID: EDOC1100038339

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