Configuring link aggregation and LACP

Overview

Ethernet link aggregation bundles multiple physical Ethernet links into one logical link, called an aggregate link. Link aggregation has the following benefits:

•Increased bandwidth beyond the limits of any single link. In an aggregate link, traffic is distributed across the member ports.

•Improved link reliability. The member ports dynamically back up one another. When a member port fails, its traffic is automatically switched to other member ports.

Basic concepts

Aggregate interface

An aggregate interface is a logical interface.

Aggregation group

An aggregation group is a collection of Ethernet interfaces. When you create an aggregate interface, the switch automatically creates an aggregation group of the same number as the aggregate interface.

Aggregation states of the member ports in an aggregation group

A member port in an aggregation group can be in either of the following states:

•Selected—A Selected port can forward user traffic.

•Unselected—An Unselected port cannot forward user traffic.

The port rate of an aggregate interface equals the total rate of its member ports in Selected state, and its duplex mode is the same as that of the selected member ports.

For more information about the states of member ports in an aggregation group, see "Static aggregation mode" and "Dynamic aggregation mode."

LACP

The Link Aggregation Control Protocol (LACP) is defined in IEEE 802.3ad. It uses LACPDUs to exchange aggregation information between LACP-enabled devices.

LACP is automatically enabled on member ports in a dynamic aggregation group. An LACP-enabled port sends LACPDUs to notify the remote system (the partner) of its system LACP priority, system MAC address, LACP port priority, port number, and operational key. Upon receiving an LACPDU, the peer port compares the received information with the information received on other member ports. In this way, the two systems reach an agreement on which ports are placed in Selected state.

Operational key

When aggregating ports, link aggregation control automatically assigns each port an operational key based on port attributes, including the port rate, duplex mode, and link state configuration.

In an aggregation group, all Selected ports are assigned the same operational key.

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