Spanning Tree Protocol

Spanning Tree Protocol

Introduction

When multiple paths exist on a network, Spanning Tree Protocol (STP) configures the network so that a switch uses only the most efficient path. The following topics are discussed in this chapter:

•Overview

•Bridge Protocol Data Units (BPDUs)

•Spanning Tree Group (STG) configuration guidelines

•Multiple Spanning Trees

Overview

Spanning Tree Protocol (STP) detects and eliminates logical loops in a bridged or switched network. STP forces redundant data paths into a standby (blocked) state. When multiple paths exist, STP configures the network so that a switch uses only the most efficient path. If that path fails, STP automatically sets up another active path on the network to sustain network operations.

The switch supports IEEE 802.1d Spanning Tree Protocol for STG 1, and Per VLAN Spanning Tree Protocol (PVST+) for STGs 2-128, by default.

NOTE: The switch also supports IEEE 802.1w Rapid Spanning Tree Protocol and IEEE 802.1s Multiple Spanning Tree Protocol. For more information, see the “RSTP and MSTP” chapter in this guide.

Bridge Protocol Data Units

To create a spanning tree, the application switch generates a configuration Bridge Protocol Data Unit (BPDU), which it then forwards out of its ports. All switches in the Layer 2 network participating in the spanning tree gather information about other switches in the network through an exchange of BPDUs.

A BPDU is a 64-byte packet that is sent out at a configurable interval, which is typically set for two seconds. The BPDU is used to establish a path, much like a hello packet in IP routing. BPDUs contain information about the transmitting bridge and its ports, including bridge and MAC addresses, bridge priority, port priority, and port path cost. If the ports are tagged, each port sends out a special BPDU containing the tagged information.

The generic action of a switch on receiving a BPDU is to compare the received BPDU to its own BPDU that it will transmit. If the received BPDU has a priority value closer to zero than its own BPDU, it will replace its BPDU with the received BPDU. Then, the application switch adds its own bridge ID number and increments the path cost of the BPDU. The application switch uses this information to block any redundant paths.

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