3Com Switch 8800 Advanced Software V5 Configuration Guide

96 CHAPTER 11: MSTP CONFIGURATION

■ Root path cost

■ Designated bridge ID (in the form of device priority)

■ Designated port ID (in the form of port ID)

■ Initial state

Upon initialization of a device, each port generates a configuration BPDU with

itself as the root, in which the root path cost is 0, designated bridge ID is the

device ID, and the designated port is the local port.

■ Selection of the optimum configuration BPDU

Each device sends out its configuration BPDU and receives configuration BPDUs

from other devices.

The process of selecting the optimum configuration BPDU is as follows:

Rules for configuration BPDU comparison:

■ The configuration BPDU that has the lowest root bridge ID has the highest

priority.

■ For configuration BPDUs with the same root bridge ID, they will be compared

by their root path costs. If the root path cost in a configuration BPDU plus the

path cost corresponding to this port is S, the configuration BPDU with the

smallest S value has the highest priority.

■ For configuration BPDUs with the same root bridge ID and the same root path

cost, they will be compared by their designated bridge IDs, then their

designated port IDs, and then the IDs of the ports through which they are

received. The smaller the ID, the higher the message priority.

■ Selection of the root bridge

At network initialization, each STP-compliant device on the network assumes itself

to be the root bridge, with the root bridge ID being its own device ID. By

exchanging configuration BPDUs, the devices compare one another’s root bridge

ID. The device with the smallest root bridge ID is elected as the root bridge.

■ Selection of the root port and designated ports

Tabl e 9 Selection of the optimum configuration BPDU

Step Description

1 Upon receiving a configuration BPDU on a port, the device performs the following

processing:

■ If the received configuration BPDU has a lower priority than that of the

configuration BPDU generated by the port, the device will discard the received

configuration BPDU without doing any processing on the configuration BPDU of

this port.

■ If the received configuration BPDU has a higher priority than that of the

configuration BPDU generated by the port, the device will replace the content of

the configuration BPDU generated by the port with the content of the received

configuration BPDU.

2 The device compares the configuration BPDUs of all the ports and chooses the

optimum configuration BPDU.