LACP

What Is LACP?

LACP is a data link layer protocol defined in IEEE 802.3ad standard. It provides a method to control the bundling of several physical ports together to form a single logical channel. It enables a network device to negotiate an automatic bundling of links by sending LACP packets to the peer. In practice, LACP protocol serves the general principle of link aggregation, which describes the effort of setting up parallel network structures to provide redundancy, or to improve performance. FS S3900 series Gigabit Ethernet switches can support LACP protocol. However, they don't support entering the aggregation port to configure various parameters. They must be configured through the interface range command.

Why Do We Need LACP?

LACP has the following advantages：

1. Increased bandwidth: The maximum bandwidth of an aggregated link is the sum of the bandwidths of all links in the link aggregation group. Compared with a single link, the link bandwidth is greatly expanded.

2. Increase reliability: A link aggregation group is composed of multiple links. When a single link fails, the traffic transmitted on this link will automatically switch to other active links, and there will be no transmission failure phenomenon.

3. Load balancing: Traffic will be allocated to multiple member links for transmission according to certain rules, improving link utilization.

4. Dynamically configurable: In the absence of network managers or manual configuration, the link aggregation group can flexibly adjust the selected or unselected status of the ports of the aggregation members based on information about the peer and local end.

How Does LACP Work?

The LACP mechanism operates by dispatching LACPDU messages between peers to synchronize and form a functioning link aggregation group. This process involves several steps:

Upon initiation of the LACP protocol, network devices such as switches engage in a handshake by exchanging LACPDU packets.

These devices determine the primary active device by evaluating system priorities and MAC addresses; the device with the lower system priority takes precedence as the active device, or if priorities are equal, the device with the lower MAC address assumes the active role.

Subsequently, the active ports are chosen based on port priority, with preference given to ports with lower priority values. If port priorities are tied, the port with the lower numerical identifier is elected as the active port. Once both ends of the connection agree on the active ports, they form an active link aggregation group, which then commences with balanced data transmission across the active links.

How to configure LACP on FS switch?

Before configuring LACP, make sure the connections between switches are successful. Then, log in to the switch through the console port and enter the CLI interface for command configuration. Typically, LACP configuration can be achieved through static and dynamic methods. The specific configuration steps are as follows.

Static configuration mainly involves four steps:

1. Create a port-channel and configure it in LACP mode for link aggregation.

2. Add member ports to the channel-group.

3. Create VLANs and add ports to VLANs.

4. Verify the LACP configuration.

Dynamic configuration mainly involves six steps:

1. Add member ports to the channel-group.

2. Set system priority to determine the active end for selecting active ports based on the priority of the active port.

3. Configure the upper threshold for the number of active interfaces to ensure network reliability while maintaining bandwidth. (This step is only applicable to CLI dynamic configuration commands)

4. Configure LACP port priorities and determine active link ports to select higher priority ports as active ports. (This step is only applicable to CLI dynamic configuration commands)

5. Create VLANs and add ports to VLANs. (This step is only applicable to CLI dynamic configuration commands)

6. Verify the LACP configuration.