3Com Switch 4200G Configuration Guide

MSTP Overview 117
Implementation of MSTP MSTP divides a network into multiple MST regions at Layer 2. The CST is generated
between these MST regions, and multiple spanning trees (or, MSTIs) can be generated
in each MST region. As well as RSTP, MSTP uses configuration BPDUs to generate
spanning trees. The only difference is that the configuration BPDUs for MSTP carry the
MSTP configuration information on the switches.
Generating the CIST
Through configuration BPDU comparing, the switch that is of the highest priority in
the network is chosen as the root of the CIST. In each MST region, an IST is figured
out by MSTP. At the same time, MSTP regards each MST region as a switch to figure
out the CST of the network. The CST, together with the ISTs, forms the CIST of the
network.
Generating an MSTI
In an MST region, different MSTIs are generated for different VLANs depending on
the VLAN-to-spanning-tree mappings. Each spanning tree is figured out
independently, in the same way as STP/RSTP.
Implementation of STP algorithm
In the beginning, each switch regards itself as the root, and generates a configuration
BPDU for each port on it as a root, with the root path cost being 0, the ID of the
designated bridge being that of the switch, and the designated port being itself.
1 Each switch sends out its configuration BPDUs and operates in the following way
when receiving a configuration BPDU on one of its ports from another switch:
■ If the priority of the configuration BPDU is lower than that of the configuration
BPDU of the port itself, the switch discards the BPDU and does not change the
configuration BPDU of the port.
■ If the priority of the configuration BPDU is higher than that of the configuration
BPDU of the port itself, the switch replaces the configuration BPDU of the port
with the received one and compares it with those of other ports on the switch to
obtain the one with the highest priority.
2 Configuration BPDUs are compared as follows:
■ The smaller the root ID of the configuration BPDU is, the higher the priority of the
configuration BPDU is.
■ For configuration BPDUs with the same root IDs, the comparison is based on the
path costs. Suppose S is the sum of the root path cost and the corresponding path
cost of the port. The less the S value is, the higher the priority of the configuration
BPDU is.
■ For configuration BPDUs with both the same root ID and the same root path cost,
the designated bridge ID, designated port ID, the ID of the receiving port are
compared in turn.
3 A spanning tree is figured out as follows:
■ Selecting the root bridge
The root bridge is selected by configuration BPDU comparing. The switch with the
smallest root ID is chosen as the root bridge.
■ Selecting the root port
For each switch (except the one chosen as the root bridge) in a network, the port
that receives the configuration BPDU with the highest priority is chosen as the root
port of the switch.
■ Selecting the designated port