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Almost every EtherChannel that fails to form does so because of a configuration mismatch. The protocol is fine. The physical layer is fine. Two member ports have different allowed VLANs, or one is access and the other is trunk, and the bundle silently refuses to include the outlier. Every engineer I've worked with has spent at least one afternoon on this.
| Mismatch | Failure mode | Visibility |
|---|---|---|
| Speed mismatch | Lower-speed link won't bundle | Logged, port shows (I) |
| Duplex mismatch | Link may bundle but performance suffers | Silent until complaints |
| Access/trunk mismatch | Member won't bundle | Logged, port shows (I) |
| Allowed VLAN mismatch | Port bundles but drops some VLANs | Silent — very hard to diagnose |
| Native VLAN mismatch | Port bundles but traffic leaks/misroutes | Silent — dangerous |
| Protocol mismatch | No bundle forms at all | Silent unless logs checked |
| LACP mode mismatch | Depends on combination — often no bundle | Silent |
The dangerous ones are VLAN mismatches. The port joins the bundle because the physical and mode configs match. But VLAN 30 flows on some members and not others, depending on how the switch picks the hash. Traffic becomes intermittent and unpredictable — worst kind of failure.
Finish the lesson once you have worked through the material. This awards ★ 35 XP.
Select each vulnerable line, then classify the finding.
Findings
Review target: 12 lines · 1 expected finding