Configuring MSTP

Overview

Spanning tree protocols eliminate loops in a physical link-redundant network by selectively blocking redundant links and putting them in a standby state.

The recent versions of STP include the Rapid Spanning Tree Protocol (RSTP) and the Multiple Spanning Tree Protocol (MSTP).

Introduction to STP

STP was developed based on the 802.1d standard of IEEE to eliminate loops at the data link layer in a LAN. Networks often have redundant links as backups in case of failures, but loops are a very serious problem. Devices running STP detect loops in the network by exchanging information with one another, and eliminate loops by selectively blocking certain ports to prune the loop structure into a loop-free tree structure. This avoids proliferation and infinite cycling of packets that would occur in a loop network.

In the narrow sense, STP refers to IEEE 802.1d STP. In the broad sense, STP refers to the IEEE 802.1d STP and various enhanced spanning tree protocols derived from that protocol.

STP protocol packets

STP uses bridge protocol data units (BPDUs), also known as configuration messages, as its protocol packets. This chapter uses BPDUs to represent all types of spanning tree protocol packets.

STP-enabled network devices exchange BPDUs to establish a spanning tree. BPDUs contain sufficient information for the network devices to complete spanning tree calculation.

STP uses the following types of BPDUs:

•Configuration BPDUs—Used for calculating a spanning tree and maintaining the spanning tree topology.

•Topology change notification (TCN) BPDUs—Used for notifying the concerned devices of network topology changes.

Configuration BPDUs contain sufficient information for the network devices to complete spanning tree calculation. Important fields in a configuration BPDU include the following:

•Root bridge ID—Consisting of the priority and MAC address of the root bridge.

•Root path cost—Cost of the path to the root bridge denoted by the root identifier from the transmitting bridge.

•Designated bridge ID—Consisting of the priority and MAC address of the designated bridge.

•Designated port ID—Consisting of the priority and global port number of the designated port.

•Message age—Age of the configuration BPDU while it propagates in the network.

•Max age—Maximum age of the configuration BPDU stored on a device.

•Hello time—Configuration BPDU transmission interval.

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