Bridge

A bridge is a Data Link layer device that connects two or more LAN segments and forwards frames selectively, based on destination MAC address, rather than blindly repeating everything like a hub. It learns which MAC addresses live on which port by observing the source address of incoming frames, building a forwarding table automatically. This reduces collisions (by segmenting collision domains) and filters unnecessary traffic from crossing into segments where it isn't needed.

Spanning Tree Protocol (STP)

When bridges (or switches) are interconnected with redundant links (for fault tolerance), this can accidentally create loops in the network topology. Loops are dangerous at the Data Link layer because:

• Frames (especially broadcast frames) can circulate endlessly, since Ethernet frames have no "hop count" / TTL field to expire them.
• This causes a broadcast storm, exponentially multiplying traffic and quickly overwhelming the network.
• The bridging/MAC address table can also become unstable, as the same source address appears to arrive from multiple different ports in rapid succession.

The Spanning Tree Protocol solves this by having bridges cooperatively compute a loop-free logical topology (a spanning tree) that still uses every bridge, but disables (blocks) redundant links that would otherwise create a loop — while keeping them on standby.

How it works, briefly:

1. Bridges exchange special messages called BPDUs (Bridge Protocol Data Units) to share topology information.
2. One bridge is elected as the root bridge (typically the one with the lowest bridge ID).
3. Every other bridge computes the shortest path (least cost) to the root bridge, designating one port as its root port.
4. For each network segment, one bridge is chosen as the designated bridge, and its port toward that segment is put in forwarding state; all other redundant ports on that segment are put into blocking state (not forwarding data, but still listening for topology changes).
5. If an active link fails, STP automatically recalculates and activates a previously blocked (backup) link to restore connectivity — providing both loop prevention and fault tolerance.