This is a role-reading sequence, not a physical topology. Each non-root bridge selects a root port; alternate paths can be intentionally non-forwarding.

Quick reference

Item	Interpretation	Operational check
Root election	Lowest bridge ID wins	Priority plus system-ID extension / MAC
Root port	Best path toward root	Compare cost and tie-breakers
Designated	Selected port for segment	Normally forwards when converged
Alternate	Other path to root	Discarding can be healthy
RSTP states	Discarding / learning / forwarding	Role and state are different
Classic timers	Hello 2s / max age 20s / delay 15s	Defaults, not a convergence guarantee
MST region	Name, revision, VLAN mapping	Must agree for same region

What to understand first

Root placement should follow the intended traffic design. An accidental low bridge priority can move the root and redirect traffic without a physical link failure.

Classic STP uses blocking, listening, learning and forwarding states; RSTP combines non-forwarding states as discarding and uses rapid transition mechanisms where applicable.

Edge/PortFast settings are for intended endpoint edges. BPDU Guard and root protection serve different purposes; investigate their trigger before restoring a port.

MST maps VLANs into instances. A mismatch in name, revision or mapping creates a region boundary even when the cables and VLAN lists match.

Commands and interpretation

Inspection commands are read-only unless explicitly labeled otherwise. Capture commands start collection; configuration-mode commits change device state.

Cisco IOS XE

Read-only EXEC

show spanning-tree summary
show spanning-tree vlan 30
show spanning-tree inconsistentports
show spanning-tree mst configuration

Inspect: Check root identity, role/state, inconsistent ports and the actual MST mapping.

Junos

Read-only operational

show spanning-tree bridge
show spanning-tree interface

Inspect: Inspect root ID, root port, costs and role/state. Match the output to RSTP, MSTP or VSTP in use.

Worked example · Illustrative, not a device capture

Illustrative role/state observations

Root ID: distribution switch A
Uplink 1: Root / Forwarding
Uplink 2: Alternate / Discarding
Root changes during incident: 0

A non-forwarding alternate can be correct loop prevention. Confirm the topology before trying to make both uplinks forward.

If the expected root changes repeatedly, investigate BPDUs, link instability and unauthorized switches.

Troubleshooting sequence

Identify the intended and actual root for the affected VLAN/instance.
Compare roles and states on both ends of the suspected segment.
Review topology changes, error counters and protection-triggered ports with timestamps.
For MST, compare region parameters and VLAN-to-instance mappings exactly.
Use a planned redundancy test; verify reconvergence and application recovery before closing.

Common mistakes

Disabling STP to make a blocked link pass traffic.
Applying edge settings to an inter-switch link without the intended design.
Assuming all vendors run an identical per-VLAN tree by default.

Acceptance checks

Actual roots and region boundaries match the design.
No unexplained inconsistent ports or sustained topology-change bursts.
Approved link-failure test meets the application recovery target.

Primary references

Vendor documentation and protocol specifications support this guide. The diagrams, scenarios and troubleshooting sequences are Wolex-authored.

STP, RSTP & MST field reference