LACP

LACP, or Link Aggregation Control Protocol, is a network protocol used to control the aggregation of multiple physical links into a single logical link, called a port-channel or link aggregation group (LAG). LACP is defined by IEEE standards for link aggregation (originally 802.3ad and later incorporated into 802.1AX). It enables increased bandwidth and redundancy between devices such as switches or servers and their uplink peers.

LACP works by exchanging Link Aggregation Control Protocol Data Units (LACPDUs) between neighboring devices to negotiate which physical links belong to the same LAG. Each end designates an actor and a partner, advertising the system identifier, port number, and a key that helps identify the group. Based on these messages, devices agree which alternate links to add and monitor link health. LACP supports both active and passive modes: in active mode a device proactively sends LACPDUs, while in passive mode it responds to received LACPDUs.

A LAG typically includes multiple parallel links, usually up to 16, to share traffic and provide redundancy. The distribution of traffic across member links is determined by a hashing algorithm and LACP configuration on the devices. If a link fails, LACP automatically removes it from the group and traffic is redistributed among remaining links. Proper cross-checking of partner IDs, keys, and other parameters is necessary to ensure correct operation.

LACP is widely deployed in enterprise and data center networks to simplify bandwidth scaling and improve link reliability. It can be used with Ethernet switches and servers with NICs that support LACP, and is compatible with most manufacturers' port-channel implementations, though exact features and maximums can vary by vendor.