Link aggregation is the practice of combining multiple physical network links into a single logical interface. This is commonly referred to by vendor terminology such as Port-Channel or EtherChannel.
Benefits and Use Cases
Link aggregation provides two main benefits:
- Redundancy: If one physical cable fails, traffic continues flowing over the remaining members without relying on STP (Spanning Tree Protocol) to recalculate topologies.
- Aggregate Capacity: It increases total bandwidth capacity across multiple traffic flows.
It is used extensively for Switch-to-Switch, Switch-to-Server, and Switch-to-Storage topologies.
LACP (Link Aggregation Control Protocol)
While you can statically force links together, LACP (IEEE 802.1AX, historically 802.3ad) is the standard dynamic negotiation protocol. Switches exchange LACP packets to confirm the links are connected to the same device and configured correctly before bundling them. Interfaces are typically configured in Active (proactively sending LACP) or Passive (responding to LACP) modes.
For a LAG (Link Aggregation Group) to form, member links must share consistent configurations, including speed, duplex, and VLAN settings.
IMPORTANT: The Single-Flow Limitation
Do NOT assume that a single TCP file transfer will utilize the sum of all link speeds. If you bond four 1 Gbps links, a single file transfer will generally still max out at 1 Gbps.
Load distribution across a LAG commonly uses a hashing algorithm based on Source/Destination IP and MAC addresses. Because a single TCP flow always yields the same hash, all packets for that specific flow are pinned to one specific physical member link to ensure they arrive in order.