Link aggregation overview

Link aggregation enables you to bundle multiple physical Ethernet links into a single port-channel, providing enhanced performance, redundancy, and availability.

We also refer to port-channels as link-aggregation groups (LAGs). A LAG is considered a single link by connected devices, the Spanning Tree Protocol, IEEE 802.1Q VLANs, and so on. When one physical link in the LAG fails, the other links stay up. A small drop in traffic is experienced when one link fails.

When queuing traffic from multiple input sources to the same output port, all input sources are given the same weight, regardless of whether the input source is a single physical link or a port-channel.

The benefits of link aggregation are as follows:

Increased bandwidth (The logical bandwidth can be dynamically changed as the demand changes.)
Increased availability
Load sharing
Rapid configuration and reconfiguration

Each LAG consists of the following components:

Links of the same speed.
A MAC address that is different from the MAC addresses of the LAG's individual member links.
An interface index for each link to identify the link to the neighboring devices.
An administrative key for each link. Only the links with the same administrative key value can be aggregated into a LAG. On each link configured to use LACP, LACP automatically configures an administrative key value equal to the port-channel identification number.

Two LAG types are supported:

Static LAGs—In static link aggregation, links are added into a LAG without exchanging control packets between the partner systems. The distribution and collection of frames on static links is determined by the operational status and administrative state of the link.
Dynamic LAGs—Dynamic link aggregation uses Link Aggregation Control Protocol (LACP) to negotiate the links included in a LAG. Typically, two partner systems sharing multiple physical Ethernet links can aggregate a number of those physical links using LACP. LACP creates a LAG on both partner systems and identifies the LAG by the LAG ID. All links with the same administrative key, and all links that are connected to the same partner switch become members of the LAG. LACP continuously exchanges LACP protocol data units (PDUs) to monitor the health of each member link.