Chapter 11. Protecting the Spanning Tree Protocol Topology

This chapter covers the following topics that you need to master for the CCNP BCMSN exam:

• Root Guard This section discusses how to protect the STP topology against unexpected switches advertising to become the Root Bridge.

• BPDU Guard This section covers unexpected STP advertisements on switch ports configured for PortFast, where single hosts connect.

• Loop Guard This section discusses how to protect the STP topology against the loss of BPDUs from the Root Bridge on a switch port.

• UDLD This section presents a feature to detect and protect against unidirectional, switch-to-switch links.

• BPDU Filtering This section explains how to filter BPDUs on a switch port to prevent the port from participating in STP altogether. Bridging loops are neither detected nor prevented.

• Troubleshooting STP Protection This section summarizes the commands that diagnose or verify actions to protect the topology.

Achieving and maintaining a loop-free Spanning Tree Protocol (STP) topology revolves around the simple process of sending and receiving bridge protocol data units (BPDU). Under normal conditions, with all switches playing fairly and according to the rules, a loop-free topology dynamically is determined.

This chapter discusses two basic conditions that can occur to disrupt the loop-free topology (even while STP is running):

• On a port that has not been receiving BPDUs, BPDUs are not expected. When BPDUs suddenly appear for some reason, the STP topology can reconverge to give unexpected results.

• On a port that normally receives BPDUs, BPDUs always are expected. When BPDUs suddenly disappear for some reason, a switch can make incorrect assumptions about the topology and unintentionally create loops.