Bridge Protocol Data Units BPDUs | Nortel Networks 42C4911 specification

Alteon OS Application Guide

Bridge Protocol Data Units (BPDUs)

To create a Spanning Tree, the switch generates a configuration Bridge Protocol Data Unit (BPDU), which it then forwards out of its ports. All switches in the Layer 2 network participat- ing 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 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 is better than its own BPDU, it will replace its BPDU with the received BPDU. Then, the switch adds its own bridge ID number and increments the path cost of the BPDU. The switch uses this information to block any nec- essary ports.

Determining the Path for Forwarding BPDUs

When determining which port to use for forwarding and which port to block, the GbE Switch Module uses information in the BPDU, including each bridge priority ID. A technique based on the “lowest root cost” is then computed to determine the most efficient path for forwarding.

Bridge Priority

The bridge priority parameter controls which bridge on the network is the STG root bridge. To make one switch the root bridge, configure the bridge priority lower than all other switches and bridges on your network. The lower the value, the higher the bridge priority. Use the following command to configure the bridge priority:

/cfg/l2/stg x/brg/prio

Port Priority

The port priority helps determine which bridge port becomes the designated port. In a network topology that has multiple bridge ports connected to a single segment, the port with the lowest port priority becomes the designated port for the segment. Use the following command to con- figure the port priority:

/cfg/l2/stg x/port x/prio

42C4911, January 2007

Chapter 5: Spanning Tree Group 107

42C4911 specifications

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Nortel Networks 42C4911 manual Bridge Protocol Data Units BPDUs, Bridge Priority, Port Priority

Models: 42C4911