3Com Switch 7750 Configuration Guide Guide

228 CHAPTER 29: 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 original
configuration BPDU received from the corresponding 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, the Switch 7750
Family also provides the following management functions.
■ Root bridge retaining
■ Root bridge backup
■ Root protection
■ BPDU protection
■ Loop guard
Root Bridge
Configuration
Table 154 lists MSTP-related configurations about root bridges.
Tabl e 154 Root bridge configuration
Operation Remarks Related section
MSTP configuration
Required
To prevent network topology
jitter caused by other related
configurations, you are
recommended to enable
MSTP after other related
configurations are performed.
“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”