Troubleshooting STP and EtherChannel

VLAN 1 has one blocking port; VLANs 10 and 20 have zero. That's normal in a dual-root design — VLANs 10/20 are rooted at this switch so no ports are blocked. VLAN 1 is blocked somewhere, meaning its root is a different switch. Confirm with `show spanning-tree vlan 1 | include This bridge`.

#Common STP Problems#link

• ▪Unexpected root bridge — a new switch with priority 0 or 4096 becomes root
• ▪Ports flapping between blocking and forwarding — unstable topology
• ▪High topology change count — check for flapping port
• ▪Ports stuck in listening/learning — timer waiting or loop
• ▪err-disabled from BPDU Guard — a switch was plugged into an access port

#EtherChannel Troubleshooting Workflow#link

Configure a 4-link EtherChannel with LACP. Verify with `show etherchannel summary`. Then change one member's allowed VLAN list to break the bundle. Verify one port shows (I) — standalone. Fix the config and verify all four members return to (P).