Spanning Tree

• Works at layer 2.

• We can connect switches together. Where multiple paths between switches exist, spanning-tree will only allow one of them to run.

• If the “active” path fails (spanning-tree PDU's stop traversing the link), then spanning-tree will make one of the other links “active”.

• Put some switches in different buildings, with multiple fibre links between them, and suddenly our Layer-2 network can withstand the loss of a building. Combine with VLANs for best effect.

• Sounds kinda simple, it isn't. Lots of “gotchas”.