Spanning Tree Protocol: Loop Prevention

Root Bridge

Topology

Redundant links 3

Animate BPDU convergence pulses

Network State

Root bridge

Diameter (cost)

Forwarding links

Blocked links

Root port (forwarding) Designated port (forwarding) Blocking port (loop broken)

How it works

IEEE 802.1D Spanning Tree Protocol (STP) keeps a switched Ethernet LAN loop-free even when engineers wire in redundant links for resilience. Every switch (bridge) has a Bridge ID = priority + MAC; the lowest Bridge ID network-wide becomes the root bridge. Every other switch then finds its cheapest path to the root using standard IEEE port costs by link speed:

10 Mbps  → cost 100
100 Mbps → cost 19
1 Gbps   → cost 4
10 Gbps  → cost 2

Root Path Cost(switch) = min over neighbors N of
   [ Root Path Cost(N) + cost(link to N) ]

Each switch then assigns port roles per IEEE 802.1D rules:

Root Port — the one port with the lowest root path cost back to the root (ties broken by the neighbor's lower Bridge ID). Always forwarding.
Designated Port — on every LAN segment, whichever attached switch offers the lowest root path cost owns that segment's forwarding port (the root bridge is designated on all of its own links).
Blocking Port — any other port on a segment. It receives BPDUs but drops all data traffic, breaking the physical loop into a logical tree.

Switches exchange Bridge Protocol Data Units (BPDUs) — advertising root ID and root path cost — to elect the root and agree on port roles; the animated pulses trace that flow of BPDUs outward from the root along the resulting spanning tree. Pick a different root bridge or add more redundant links and watch the tree, and which links get blocked, recompute.

Click a switch to make it the root bridge

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