EtherChannel Explained: Bundling Links for Speed & Redundancy
EtherChannel bundles several physical links into one logical link, combining their bandwidth and providing redundancy. Crucially, Spanning Tree sees the bundle as a single link, so none of the member ports get blocked — you get to use all the bandwidth.
LACP vs PAgP
- LACP (802.3ad) — the open-standard negotiation protocol; use it for multi-vendor environments. Modes: active/passive (at least one side must be active).
- PAgP — Cisco-proprietary equivalent. Modes: desirable/auto.
- You can also configure a bundle unconditionally with mode on (no negotiation) — but matching mismatches then cause problems.
Why EtherChannel beats separate links
Four separate 1 Gbps links between switches would leave three blocked by STP (loop prevention), wasting 75% of the capacity. Bundled as an EtherChannel, all four forward as one 4 Gbps logical link with automatic failover if a member dies. Load-balancing spreads traffic across members by a hash (source/destination MAC or IP).
Frequently asked questions
What is EtherChannel used for?
Bundling multiple physical links into one logical link to increase bandwidth and provide redundancy, without Spanning Tree blocking the extra links.
What is the difference between LACP and PAgP?
LACP (802.3ad) is the open standard usable across vendors; PAgP is Cisco-proprietary. Both negotiate the bundle automatically.
Does EtherChannel work with Spanning Tree?
Yes — STP treats the whole bundle as a single logical link, so all member ports can forward instead of being blocked.
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