IEEE 802.1D · BRIDGE PROTOCOL · LOOP-FREE ETHERNET

Distributed Spanning Tree
Protocol Laboratory

Watch Ethernet bridges exchange BPDUs, elect a root, choose root and designated ports, and deactivate only the redundant ports that would form loops.

Bridge ID = priority + MACBPDU comparisonRoot Port · RDesignated Port · DAlternate / Blocked · B×physical topology ≠ forwarding tree

Bridge to inspectTopology event

BPDU round0

Current root—

Blocked ports0

Protocol stateINIT

Physical topology and forwarding tree

CLICK A BRIDGE OR STEP A ROUND

R Root PortD Designated PortB× Alternate / Blocked→ BPDU message

Bridge B1

LOCAL ROOT

Port roles

Port / link	Role	State

Last BPDU inbox

Root-election history

GLOBAL BARRIER AFTER EACH ROUND

Round	BPDUs	Bridges changed	Root views	Blocked ports	Status

Ready. Every bridge initially claims itself as root.

1. Root election

Each bridge begins by advertising itself as root. The lowest Bridge ID wins. A Bridge ID is ordered first by configured priority and then by MAC address.

superior BPDU = minimum of:
(root ID, root path cost,
 sender bridge ID, sender port ID)

2. Port roles

Every non-root bridge selects exactly one Root Port. Each segment selects one Designated Port. A remaining redundant port becomes Alternate/Blocked; the bridge itself remains active.

R = best path toward root
D = best BPDU onto segment
B× = redundant path blocked

3. Loop-free forwarding

The physical cable remains connected, but blocked ports do not forward ordinary frames. In the example, B3→B2 and B4→B3 block after convergence. If a forwarding link fails, STP elects a new tree and may reactivate an alternate port.

physical links: 5
tree links: |V| - 1 = 3
redundant links retained for recovery