Cisco Spanning Tree Checks
Use these commands when a VLAN is blocked unexpectedly, a loop is suspected, or topology changes are causing instability.
Status checks
show spanning-tree summary
show spanning-tree vlan <vlan-id>
show spanning-tree blockedports
show spanning-tree rootInterface-level checks
show spanning-tree interface ethernet0/1 detail
show interfaces ethernet0/1 status
show logging | include SPANTREE|STP|LOOPWhat to look for
- Unexpected root bridge.
- Ports blocking in the wrong location.
- Frequent topology changes.
- Missing PortFast on access ports.
- BPDU Guard or Root Guard events.