LACP

What is LACP?

Link Aggregation Contro Protocol (LACP) is a protocol standardized by IEEE 802.3ad to implement dynamic link aggregation groups (LAGs). LACP enables multiple physical links among switches into a single logically virtual link. When a member link in the LACP group fails, the traffic initially transmitted from the link will be switched to forward from another active link, thus playing the role of redundant link and ensuring interrupted business when a network failure occurs.

How Does LACP Work?

LACP is achieved through the following four working processes:

Exchange LACPDU Messages:

Devices create an Eth-Trunk and configure it for LACP. They then exchange LACPDU messages, which include information like system priority and MAC addresses.

Determine the Active Device:

Each device receives LACPDU messages from the other. The device with the higher system priority becomes the active one. If priorities are the same, the device with the lower MAC address will be selected as the active device.

Select Active Links:

The active device determines which links are active based on their priority and interface numbers. If the number of active links reaches a set limit, additional links are used as backups. For example, if the limit is 2, only the top 2 links with the highest priority are active, while others are backup links.

Link Failover:

If an active link fails, LACP automatically selects a backup link with the highest priority to replace it, ensuring that data continues to flow reliably.

This process ensures efficient load balancing and fault tolerance in network connections.

LACP vs LAG

Link Aggregation Group (LAG), or Eth-Trunk, is a logically virtual link formed by multiple physical links. Within an LAG, all member interfaces have only a single logical interface, an aggregated LAG interface, or an Eth-Trunk interface. The LAG interface can be viewed as a physical Ethernet interface that is responsible for selecting the member interface for data forwarding.

LACP is used to enable a LAG to automatically configure network switch ports, isolate link failures, and activate failover. Compared to LACP, another method to achieve LAG is manual configuration, which means the LAG will not enable any link aggregation protocol and the establishment of Eth-Trunk and the addition of member interfaces are manually configured. The following table lists the main differences between LACP and munal configuration for forming LAG.

Compared with Static LAG, What are the Advantages of LACP?

Compared to static LAG, dynamic LACP offers several advantages:

Automatic Link Management: All links are active and share traffic in static mode. If a link fails, the remaining links automatically take over the load. LACP, however, differentiates between active and backup links, ensuring higher reliability and better load balancing. For example, the blue line refers to M active links, and the red line means N redundant inactive links. The whole bandwidth in the LAG is the sum of the M active links. However, the maximum bandwidth provided in the LAG is M + N. If an active link fails, a redundant red link is activated to maintain performance, which maximizes link utilization and ensures better load balancing.

Enhanced Fault Detection: Static LAG can only detect limited faults like link disconnections. LACP can detect broader issues, including link failures and misconfigurations. For example, if device A is mistakenly connected to the wrong device C, LACP identifies and corrects it to ensure data is sent to the intended device B. While in manual mode, if the Eth-Trunk can not detect the fault, then the package designed to be forwarded to device B may be only sent to device C by mistake.