Link Aggregation
Link Aggregation in Manual Load Balancing Mode
Link aggregation can work in manual load balancing mode and LACP mode.
In manual load balancing mode, you need to manually create an Eth-Trunk interface and add member interfaces to the Eth-Trunk interface, without the assistance of the LACP protocol. In this mode, all the member interfaces of an LAG share the traffic evenly. If an active link fails, the other active links share the traffic evenly. If a high link bandwidth between two directly connected devices is required but the peer device does not support the LACP protocol, you can use the manual load balancing mode.
Link Aggregation in LACP Mode
Background
Eth-Trunk in manual load balancing mode, as a link aggregation technology, can increase the bandwidth. However, this mode can only detect link disconnections, but cannot detect other faults such as link layer faults and incorrect link connections.
The Link Aggregation Control Protocol (LACP) is used, which can improve fault tolerance of the Eth-Trunk and ensure high reliability of the member links.
LACP uses a standard negotiation mechanism for switching devices, ensuring that switching devices automatically create and enable aggregated links based on their configurations. After aggregated links are created, LACP maintains link status. If an aggregated link's status changes, LACP automatically adjusts or disables the link.
For example, in Figure 1 an Eth-Trunk link should be established between DeviceA and DeviceB by bundling four interfaces on DeviceA into an Eth-Trunk interface and connecting it to the corresponding interfaces on Device B. However, one of the interfaces is incorrectly connected to the interface on DeviceC. As a result, the Eth-Trunk interface cannot detect the fault in time and continues sending data to DeviceC.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 1 Incorrect Eth-Trunk connection
Concepts
LACP system priority
LACP system priorities are set on devices at both ends of a trunk link. In LACP mode, active member interfaces selected by both devices must be consistent; otherwise, the LAG cannot be established. To keep active member interfaces consistent at both ends, set a higher priority for one end. In this manner, the other end selects active member interfaces based on the selection of the peer. The smaller the LACP system priority value, the higher the LACP system priority.
LACP interface priority
The LACP interface priority is set for a member interface to determine whether it can be selected as an active member interface. The smaller the LACP interface priority value, the higher the LACP interface priority.
M:N backup
In LACP mode, LACP is used to negotiate parameters to determine active member 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 load balancing to be carried out across M active links.
As shown in Figure 2, 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, load balancing is performed on the M active links; no data is transmitted over the 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 the N backup links to replace the faulty link. In such a situation, the actual bandwidth of the aggregated link is still the sum of M links'bandwidth; the maximum bandwidth of the aggregated link, however, becomes the sum of the M+N-1 links'bandwidth.
Figure 2 M:N backup network diagram
M:N backup is mainly applied in situations where the bandwidth of M links must be assured, and a fault tolerance mechanism in place. If an active link fails, the system selects the backup link with the highest priority and this backup link becomes 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.
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