PlantUML DiagramEtherChannel Bundle Across Two Switches
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| Approach | Links used | Total bandwidth | STP state |
|---|---|---|---|
| Single uplink | 1 of 1 | 1 Gbps | Forwarding |
| Two uplinks, no bundle | 1 of 2 (STP blocks one) | 1 Gbps | 1 FWD, 1 BLK |
| Two uplinks, EtherChannel | 2 of 2 | 2 Gbps (aggregate) | Both FWD |
That table is the whole argument for EtherChannel. Without it, adding a second uplink gives you redundancy — but STP blocks the second link and you don't get any extra bandwidth. With a bundle, both links forward traffic and you get both redundancy and double the throughput.
A Port-Channel is a logical interface that aggregates multiple physical links. From the switch's perspective, the bundle is one interface for STP purposes. To the rest of the network, the bundle behaves like a single high-bandwidth link.
STP treats the Port-Channel as one port. That's the key. Without the bundle, STP blocks three of those four links. With the bundle, all four forward, and STP's view of the topology is simpler — one link instead of four.
EtherChannel load-balances by flow, not by packet. Each frame is assigned to a physical link based on a hash of some header fields — source MAC, destination MAC, source IP, destination IP, or a combination. All frames of a given flow go out the same physical link, which preserves ordering.
The hash method matters. A single file transfer between two hosts is one flow — it'll use one physical link and only get 1 Gbps of the aggregate. The 4 Gbps is only achieved when you have many simultaneous flows. This trips up people who expect a single stream to hit full aggregate bandwidth.
The default hash on Cisco switches is often source-destination IP. For traffic between two routers, that produces a small number of flows. For traffic between many clients and a server, it produces many flows and distributes well. Check what your switch is configured for and adjust if needed.
EtherChannel bundles physical links into one logical interface. All links forward, bandwidth aggregates, STP treats the bundle as one link. Next lesson covers the negotiation protocols that make the bundle work: LACP and PAgP.
Finish the lesson once you have worked through the material. This awards ★ 30 XP.