3Com 7700 Configuration Manual page 235

Configuring the MSTP
Running Mode
Perform the following configuration in system view.
Table 21 Specify the Switch as Primary or Secondary Root Switch
Operation
Specify current switch as the primary root
switch of the specified spanning tree.
Specify current switch as the secondary root
switch of the specified spanning tree.
Specify current switch not to be the primary or
secondary root.
After a switch is configured as primary root switch or secondary root switch, you
cannot modify the bridge priority of the switch.
You can configure the current switch as the primary or secondary root switch of
the STI (specified by the instance instance-id parameter). If the instance-id takes
0, the current switch is specified as the primary or secondary root switch of the
CIST.
The root types of a switch in different STIs are independent of one another. The
switch can be a primary or secondary root of any STI. However, it cannot serve as
both the primary and secondary roots of one STI.
If the primary root is down or powered off, the secondary root will take its place,
unless you configure a new primary root. Of two or more configured secondary
root switches, MSTP selects the one with the smallest MAC address to take the
place of the failed primary root.
When configuring the primary and secondary switches, you can also configure the
network diameter and hello time of the specified switching network. For detailed
information, refer to the configuration tasks "Configuring the Switching Network
Diameter" and "Configuring the Time Parameters of a Switch".
Note: You can configure the current switch as the root of several STIs, however, it
is not necessary to specify two or more roots for an STI. In other words, please do
not specify the root for an STI on two or more switches.
You can configure more than one secondary root for a spanning tree through
specifying the secondary STI root on two or more switches.
Generally, you are recommended to designate one primary root and more than
one secondary roots for a spanning tree.
By default, a switch is neither the primary root or the secondary root of the
spanning tree.
MSTP and RSTP are compatible and they can recognize the packets of each other.
However, STP cannot recognize MSTP packets. To implement the compatibility,
MSTP provides two operation modes, STP-compatible mode and MSTP mode. In
STP-compatible mode, the switch sends STP packets by every port and serves as a
region itself. In MSTP mode, the switch ports send MSTP or STP packets (when
Configuring MSTP
Command
stp instance instance-id root primary [
bridge-diameter bridgenum [ hello-time
centi-senconds ] ]
stp instance instance-id root secondary [
bridge-diameter bridgenum [ hello-time
centi-senconds ] ]
undo stp instance instance-id root
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