Why does EtherChannel keep both links active when STP normally blocks one?

Question

I built the classic setup in the lab: two switches with **two** cables between them. Before configuring anything, spanning tree put one of the ports into blocking — makes sense, it's preventing a loop.

Then I bundled the two ports into an EtherChannel (`channel-group 1 mode active` on both sides) and suddenly *both* links are forwarding and STP isn't blocking either one. Aggregate throughput doubled.

What I don't get: the physical loop is still there — two cables between the same two switches. So why doesn't STP block one member like it did before? How does bundling make a genuine parallel path suddenly loop-safe? And is LACP doing something special here, or is it just about how STP views the port-channel?

ben31833 · Accepted Answer

The key insight: **STP runs on the logical port-channel, not the physical members.** Once two ports bundle into `Port-channel1`, spanning tree sees a *single* interface. There's no parallel path to block, so every member link forwards. You get the aggregate bandwidth AND STP loop-freedom at the same time.

Why it's safe: internally the port-channel never loops a frame back out the member it arrived on, and load balancing is **per-flow**, not per-packet. A hash (src/dst MAC by default, or IP/L4) pins each conversation to one member, so frames stay in order.

Bonus STP effect: the bundle's path cost drops because cost is derived from bandwidth. Two 1 Gbps links bundled look like ~2 Gbps, so the port-channel is *more* likely to become the root/designated path than a single link would be.

Standard negotiation is **LACP (802.1AX/802.3ad)**:
```
interface range g0/1 - 2
 channel-group 1 mode active
!
interface port-channel1
 switchport mode trunk
```
Mode combos that actually form: active/active or active/passive. `passive/passive` never bundles (nobody initiates). All members must match speed/duplex, trunk/access mode, allowed VLANs, and native VLAN or the bundle won't come up. In the lab, run `show etherchannel summary` and `show spanning-tree` — you'll see one Po1 in the STP topology instead of two blocked ports.

Shira Sivroni · Answer

Adding the troubleshooting angle, since a bundle that won't form is the most common lab issue.

If both physical ports stay independent (STP then blocks one, and you're confused why), check for a config mismatch. LACP requires the member ports to be consistent on:
- speed & duplex
- switchport mode (both access or both trunk)
- allowed VLAN list + native VLAN on trunks
- STP settings on the interface

Any mismatch and the port drops out of the channel — sometimes err-disabled. `show etherchannel summary` shows a flag per port: `P` = bundled, `s`/`I` = suspended/independent, `D` = down. If you see `I`, LACP isn't negotiating (often passive/passive, or one side is a plain access port).

Also remember a channel is only as loop-safe as its config: never leave one member outside the channel-group, or you *do* create a real parallel path and STP will (correctly) block it while the bundle carries traffic — messy. Configure all members into the group first, then bring the interfaces up.

Why does EtherChannel keep both links active when STP normally blocks one?

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