Link Aggregation and Redundancy
Link aggregation (LACP 802.3ad / static) bundles links into one logical interface for bandwidth and redundancy. LACP negotiates via LACPDUs; static (mode on) requires manual matching and offers no negotiation safety. Hashing distributes flows across members-typically based on L2/L3/L4 fields. A single flow uses one member; aggregate bandwidth is not per-flow.
Traps: (1) All member links must match speed, duplex, and VLAN config. (2) Mismatched LACP modes (active/passive vs on) fail to bundle. (3) A single flow cannot exceed one member's bandwidth-do not claim N×Gbps per flow. (4) Hashing asymmetry can cause out-of-order or uneven load. (5) Spanning tree sees the bundle as one link-no STP blocking within a bundle.
Rule of thumb: use LACP active on both ends for negotiation and failure detection; verify member state and hashing distribution.
Worked example: Two 10G links bundled = 20G aggregate, but one large TCP flow still tops out near 10G. To use both, ensure multiple flows with varied hash inputs (source/dest IP, port). Verify with per-member counters-if one member is idle, hashing is skewed.