Link Aggregation Explained: Beyond a Single Cable
Link aggregation combines multiple physical links between two devices into one logical link — adding their bandwidth together and surviving individual cable failures. In the Cisco world it's implemented as EtherChannel, negotiated by the standard LACP protocol.
What it solves
One 1 Gbps uplink between switches becomes a bottleneck — but adding a second cable without aggregation just gets it blocked by Spanning Tree (loop prevention). Aggregating the two into one logical link gives you 2 Gbps and failover, with STP seeing a single loop-free connection.
How load balancing actually works
Traffic isn't split per-packet — each flow (conversation) is hashed onto one member link based on MAC/IP/port values. This keeps packets of a flow in order, but means a single large transfer uses only one member's bandwidth — aggregation shines with many flows, not one big one. A common surprise worth knowing before an interview asks it.
Common mistakes
Mismatched member settings (speed, duplex, VLANs, trunk mode) prevent bundling — configure via the port-channel interface so members stay identical. And both ends must run compatible modes (LACP active/active or active/passive). Verify with show etherchannel summary — bundled ports show (P), failures show (I).
Frequently asked questions
What is link aggregation?
Combining multiple physical links between two devices into a single logical link, adding bandwidth and providing automatic failover if a member fails.
Does link aggregation double the speed of a single transfer?
Usually not — load balancing assigns each flow to one member link, so a single large transfer uses one link's bandwidth. Aggregate capacity benefits many simultaneous flows.
What protocol negotiates link aggregation?
LACP (802.3ad) is the open standard; Cisco also has proprietary PAgP. At least one side must actively negotiate for a bundle to form.
Why doesn't Spanning Tree block aggregated links?
STP sees the whole bundle as one logical link, so there's no loop between the two devices — all member links forward simultaneously.
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