Aliases: LACP, 802.3ad
Link Aggregation Control Protocol (LACP)
The negotiation protocol that bonds multiple physical links into one logical pipe — more aggregate bandwidth and instant failover, with per-flow hashing as the catch.
What is LACP?
LACP (Link Aggregation Control Protocol, 802.1AX) lets two devices bundle several physical Ethernet links into one logical interface — a port channel or bond. LACP is the handshake layer: both ends exchange LACPDUs to verify each member link truly connects the same pair of devices, then continuously monitor membership, ejecting failed or miswired links from the bundle within seconds. The alternative — static aggregation without LACP — forwards happily into miscabled or half-dead links, which is why “always LACP, never static” is standing advice.
The hashing caveat that surprises everyone
An aggregate of four 10G links is not a 40G pipe for any single flow. Traffic is distributed per-flow by hashing (MAC/IP/port tuples), so one elephant flow — a database replication stream, a bulk backup — rides exactly one member link and caps at 10G while siblings idle. Aggregate bandwidth is real only across many flows. Corollary: uneven hash distributions happen (few clients, aligned ports), and monitoring must watch per-member utilization, not the bundle average that hides a saturated link inside a “half-idle” channel.
Where it appears in cloud-adjacent work
Data-center leaf switches to servers (hypervisor NIC bonding), switch-to-switch uplinks, and notably Direct Connect LAGs — AWS lets you aggregate multiple DX connections with LACP into one logical circuit, applying the same per-flow caveats at cloud-interconnect scale. MLAG/vPC variants extend bonding across two switches for chassis-level redundancy.
What people get wrong
- Selling the bundle as N× single-flow speed to application teams who then benchmark one stream.
- Mismatched hash policies on either end producing asymmetric distribution.
- Ignoring LACP timers: slow (30 s) vs fast (1 s) detection changes failover behavior meaningfully; pick deliberately.
Primary source: IEEE 802.1AX — Link Aggregation
Historical figures and technical concepts for informational purposes only. Not technical, professional, legal, or financial advice. Sources: Official Documentation.