STP prevents L2 loops by electing a Root Bridge and blocking redundant links. This chapter covers the BPDU exchange, port roles and states, the Bridge ID, and Rapid Spanning-Tree improvements.
Ethernet frames have no TTL. Send a single broadcast into a network with a loop, and switches flood it forever — multiplied each time it crosses a switch. Within seconds, every link saturates with copies of the same frame, the network collapses, and no useful traffic gets through. STP exists to make redundant L2 paths safe.
Every switch has a Bridge ID = priority (default 32768) + base MAC. The switch with the lowest Bridge ID becomes the Root Bridge. All other switches calculate their shortest path (root cost) to the root, then block any port that would create a redundant path.
| Speed | Cost (legacy 802.1D) | Cost (long 802.1t) |
|---|---|---|
| 10 Mbps | 100 | 2,000,000 |
| 100 Mbps | 19 | 200,000 |
| 1 Gbps | 4 | 20,000 |
| 10 Gbps | 2 | 2,000 |
| 100 Gbps | — | 200 |
3 more sections in this chapter, plus a 2-question quiz.
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