Chapter 12. Advanced Spanning Tree Protocol

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

• Rapid Spanning Tree Protocol (RSTP) This section discusses the enhancements that allow switches to run STP efficiently, offering fast convergence.

• Multiple Spanning Tree (MST) Protocol This section discusses the latest IEEE standard that supports a reduced number of STP instances for a campus network, while using RSTP for efficient operation.

Familiarity with the IEEE 802.1D STP standard is essential because that protocol is used universally to maintain loop-free bridged and switched networks. However, it now is considered a legacy protocol, offering topology change and convergence times that are not as acceptable as they once were.

This chapter discusses the many STP enhancements that are available in new standards. Rapid STP (RSTP) is presented first because it provides the foundation for efficient STP activity. RSTP can be coupled with either per-VLAN STP (PVST+) or Multiple STP modes. This allows a Layer 2 campus network to undergo change quickly and efficiently, with little downtime for today's applications.

This chapter also covers Multiple STP (MST or MSTP). MST allows VLANs to be individually mapped into arbitrary STP instances while RSTP operates in the background. You can use MST to greatly simplify the Layer 2 topologies and STP operations when many VLANs (and many instances of STP) are present in a network.