3Com Switch 4200G Configuration Guide

118 CHAPTER 20: MSTP CONFIGURATION
First, the switch generates a designated port configuration BPDU for each of its
port using the root port configuration BPDU and the root port path cost, with the
root ID being replaced with that of the root port configuration BPDU, root path
cost being replaced with the sum of the path cost of the root port configuration
BPDU and the path cost of the root port, the ID of the designated bridge being
replaced with that of the switch, and the ID of the designated port being replaced
with that of the port.
The switch then compares the resulting configuration BPDU with the configuration
BPDU received from the peer port on another switch. If the latter takes precedence
over the former, the switch blocks the local port and remains the port's
configuration BPDU unchanged, so that the port can only receive configuration
messages and cannot forward packets. Otherwise, the switch sets the local port to
the designated port, replaces the original configuration BPDU of the port with the
resulting one and releases it regularly.
MSTP Implementation
on Switches
MSTP is compatible with both STP and RSTP. That is, switches with MSTP employed
can recognize the protocol packets of STP and RSTP and use them to generate
spanning trees. In addition to the basic MSTP functions, S4200G series switches also
provide the following other functions for the convenience of users to manage their
switches.
■ Root bridge retaining
■ Root bridge backup
■ Root protection
■ BPDU protection
■ Loop prevention
■ Digest snooping
■ Rapid transition
Root Bridge
Configuration
Table 84 lists MSTP-related configurations about root bridges.
Table 84 Root bridge configuration
Operation Description Related section
MSTP configuration Required
To prevent network topology jitter
caused by other related
configurations, you are
recommended to enable MSTP after
performing other configurations.
MSTP Configuration
MST region
configuration
Required MST Region Configuration
Root bridge/secondary
root bridge
configuration
Required Root Bridge/Secondary Root
Bridge Configuration
Bridge priority
configuration
Optional
The priority of a switch cannot be
changed after the switch is specified
as the root bridge or a secondary root
bridge.
Bridge Priority Configuration
MSTP operation mode
configuration
Optional MSTP Operation Mode
Configuration
Maximum hops of MST
region configuration
Optional MST Region Maximum Hops
Configuration