PlantUML DiagramEtherChannel Troubleshooting Decision Tree
A structured lesson workspace with readable content, hands-on examples, and a clean path to completion.
You've just configured a 4-link EtherChannel between two switches. `show etherchannel summary` shows only 2 of the 4 members as bundled — the other 2 are in `(I)` stand-alone state. `show interfaces` shows both as up. What's the cause?
Half-bundled EtherChannels are the most common failure mode. The good news: `show etherchannel` output — read carefully — tells you exactly what's wrong. The bad news: you need to know what each flag means.
| Symptom | Likely cause | How to confirm |
|---|---|---|
| All members (I) stand-alone | Protocol mismatch, or none of them match | `show etherchannel port-channel` |
| Some members (I), some (P) | Config mismatch on the (I) members | Compare `show run int <port>` between a good and bad member |
| All members (D) down | Physical links down or not connected | `show interfaces status` |
| Port-channel exists but no members bundled | One-sided config or mode issue | Check both ends' config |
| Members show (s) suspended | Member doesn't match bundle requirements | `show etherchannel detail` |
| Bundle comes up then drops | Flapping member or LACP timeout | Logs and interface counters |
There it is. Gi0/1 allows VLANs 10, 20, 30. Gi0/3 only allows 10, 20. That mismatch means Gi0/3 doesn't match the bundle requirements — it stays in (I) state. In this case the bundle did form (with the other members), but Gi0/3 sits outside it, wasting a physical link.
! Fix Gi0/3 to match the other membersSW1(config)# interface gigabitEthernet0/3SW1(config-if)# switchport trunk allowed vlan add 30SW1(config-if)# end! The port should rejoin the bundle within secondsSW1# show etherchannel summary | include Po1
Note the use of `add` — not a bare `switchport trunk allowed vlan 30`, which would replace the current list. This is the same trap as the trunk troubleshooting lesson. Always `add` when you want to extend an existing list.
If only one side of the bundle is configured, the configured side will show as `(I)` on all members — it's sending LACPDUs but the peer isn't responding with matching info. The un-configured side just sees normal switchport traffic, with each link acting independently. STP will block three of the four links on the un-configured side, and the bundle silently has 1 Gbps capacity instead of 4.
A 4-link LACP EtherChannel has two members bundled (P) and two in (I). Configs on all four members look identical in every command you've compared. What should you check next?
Select the safest next action.
Every EtherChannel problem is either a config mismatch or a physical failure. The discipline is refusing to skip the config comparison — copy the good member's config, paste it on the bad one, and diff them line by line. It takes thirty seconds and finds the answer nine times out of ten.
Finish the lesson once you have worked through the material. This awards ★ 35 XP.