STP / RSTP Convergence Simulator

Build a network topology, run STP or RSTP, and inspect root bridge election, port states, and convergence timeline.

Add Switch
Add Link
Tip: Add at least 3 switches with redundant links (triangle or ring) to see blocked ports. Load a preset with the button below or build your own topology.

Current Topology

Switches
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Links

Topology Diagram

Root Bridge
Forwarding Port
Blocking / Discarding Port
Root Port
Designated Port
Dashed line = blocked link

Root Bridge Election

Port States

BPDU Inspector

Convergence Timeline

STP / RSTP Concepts

Root Bridge Election

The switch with the lowest Bridge ID wins. Bridge ID = Priority (default 32768) + MAC address. Ties on priority go to numerically lower MAC. Only one root bridge per VLAN.

Root Port

Every non-root switch elects one Root Port — the port with the lowest cumulative path cost to the root bridge. Ties are broken by neighbor Bridge ID, then port ID.

Designated Port

Each network segment has exactly one Designated Port — the port that offers the best (lowest cost) path to the root. All ports on the root bridge are designated.

Blocked / Discarding Ports

Any port that is neither a Root Port nor a Designated Port is blocked (STP) or discarding (RSTP). These break Layer-2 loops while maintaining a loop-free topology.

RSTP (802.1w) Edge Ports

RSTP edge ports connect to end hosts, not other switches. They transition immediately to Forwarding (no listening/learning delay), equivalent to STP's PortFast feature.

RSTP Convergence Speed

STP convergence takes 30–50 seconds (Listening 15s + Learning 15s per port). RSTP uses a Proposal/Agreement handshake, converging in under 2 seconds per hop.

Path Cost (IEEE 802.1D)

10 Gbps = 2, 1 Gbps = 4, 100 Mbps = 19, 10 Mbps = 100, 1.5 Mbps = 1000. Lower cost = preferred path. Root bridge advertises cost 0; each hop adds its port cost.

BPDU (Bridge Protocol Data Unit)

BPDUs carry: Root Bridge ID, Root Path Cost, Sender Bridge ID, Port ID, and timers. Root bridge originates BPDUs every Hello Time (2s). Other bridges relay them outbound on Designated Ports.