3Com Switch 7754, Configuration Manual

Page: 246 / 940

246 C
HAPTER
30: MSTP C
ONFIGURATION
For each switch in a network, the port through which the configuration BPDU
with the highest priority is received is chosen as the root port of the switch.
■
Determining the designated port
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, 3Com 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 guard
Root Bridge
Configuration
Table 171 lists MSTP-related configurations about root bridges.
Table 171 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” on
page 258
MST region
configuration Required “MST Region Configuration”
on page 247
Root bridge/secondary
root bridge
configuration Required “Root Bridge/Secondary Root
Bridge Configuration” on
page 249