Spanning Tree Protocol

What this chapter covers

The Spanning Tree Protocol chapter explains how redundant Layer 2 links stay useful without creating loops.

It begins with the classic protocol and the way a root bridge is elected. It then moves to the faster RSTP, to MSTP for load balancing across VLANs, and to the messages and protections that go with them.

Four syllabus topics are grouped into three pages.

After this chapter you should be able to do the following:

• Work out the root bridge, root ports and blocked ports from a diagram of priorities and costs.
• Explain why RSTP recovers faster than STP.
• Describe what makes two MSTP bridges part of the same region.
• Name the fields in a BPDU and the address it is sent to.
• Choose between edge ports, restricted role and BPDU guard to protect user ports.

How the topics fit together

Each page builds on the one before it.

STP and RSTP supply the vocabulary: root bridge, root port, designated port, alternate port, and cost. MSTP applies the same ideas to groups of VLANs. The BPDU page shows what bridges send to each other, and how to keep untrusted devices from sending it.

How to use these pages

Each topic page has the same shape, so you can skim for what you need.

• What it is: the idea in plain words.
• How it works: the mechanism, with diagrams and example commands.
• What to remember for the exam: the facts to memorise.
• Common mistakes: the classic traps.
• Check yourself: three questions to test your understanding.

Draw three switches on paper with different priorities and costs, and work out the root, the root ports and the blocked port by hand. Then build the same lab and check your answer with the monitor command on each bridge.