What are the four steps the Spanning Tree Algorithm uses to create a loop-free topology?
Select a root bridge, block redundant paths, create a loop-free topology, and recalculate on failure.
* Root bridge and least-cost paths to it. — Luca Ghio, CC BY-SA 4.0, via Wikimedia Commons. *
The four STA (Spanning Tree Algorithm) steps:
- Select a Root Bridge — one switch becomes the reference point for the entire spanning tree
- Block Redundant Paths — STP (Spanning Tree Protocol) intentionally blocks paths that could cause loops; blocked ports don't forward user data
- Create a Loop-Free Topology — blocked ports create non-forwarding links, so each switch has only a single path to the root bridge (like branches on a tree)
- Recalculate on Link Failure — if a cable or switch fails, STP recalculates and unblocks necessary ports to restore connectivity
Why this list feels redundant: these are Cisco's exact four bullets, but they aren't four independent actions. Steps 1–2 are the work STP does (pick a reference point, then block looping paths); step 3 is just the result of step 2 (a tree with one path per switch); step 4 is what happens later when the topology changes. Read it as "elect a root → block loops → (you now have a loop-free tree) → re-run if anything changes."
Key point: Recalculation also happens when a new switch or inter-switch link is added to the network — not just on failures.
Go deeper:
The Spanning-Tree Algorithm (NetworkAcademy.IO) — the elect-root → cost-to-root → block-redundant-paths sequence with diagrams.