LACP

LACP (Link Aggregation Control Protocol, IEEE 802.1AX, formerly 802.3ad) combines several physical Ethernet connections between two devices into a single logical link. That gives more total bandwidth, and if one of the bundled cables fails, traffic simply continues over the remaining ones instead of the connection dropping entirely. LACP is typically used between two switches, or between a switch and a server.

In practice, two or more ports on both sides are grouped together and configured identically; LACP actively negotiates which ports actually end up in the bundle, and automatically detects misconfiguration or a failed link. A single data transfer usually still travels over just one of the bundled links at a time — the real gain comes from many parallel connections and from resilience against a single cable failing.

A common misconception is that LACP automatically doubles the speed of a single connection, such as one large file transfer — that's usually not the case, since a single connection typically stays on one cable. Both sides of a bundle also need to be configured identically and correctly; if the setting only matches on one side, the bundle either doesn't work or network loops appear.