Chapter4.Spanning Tree

Chapter 4. Spanning Tree

The Spanning Tree Protocol (STP) is a Layer 2 protocol that prevents loops in transparently bridged networks. Due to the nature of transparent bridging, when an active looped topology exists, a network meltdown generally occurs in a matter seconds. STP is a protocol that builds a logical loop-free topology, ensuring the network does not suffer from major problems such as a broadcast storm or bridge table corruption.

STP was originally developed by Digital Equipment Corporation in 1983 to address the issues of running transparent bridging in a looped Layer 2 topology. Today, STP exists in two flavors:

• DEC The original Spanning Tree Protocol, created by Digital Equipment Corporation.

• IEEE Standards-based Spanning Tree Protocol, specified in the 802.1d standard, initially developed by Radia Perlman. This protocol was developed from the DEC STP implementation; however, the versions are incompatible with each other. The IEEE 802.1d version is almost exclusively used in today's networks.

This chapter focuses exclusively on the IEEE 802.1d version of STP. After some initial introductory material, this chapter presents the following configuration scenarios, which provide you with the practical knowledge required to implement spanning tree:

• Scenario 4-1: Configuring the Root Bridge

• Scenario 4-2: Configuring STP Load Sharing

• Scenario 4-3: Configuring Root Guard

• Scenario 4-4: Configuring Spanning Tree PortFast

• Scenario 4-5: Configuring PortFast BPDU Guard

• Scenario 4-6: Configuring PortFast BPDU Filter

• Scenario 4-7: Configuring UplinkFast

• Scenario 4-8: Configuring BackboneFast

• Scenario 4-9: Improving Convergence and Load Sharing by Using a Multilayer Topology

• Scenario 4-10: Troubleshooting Spanning Tree