Configuring The Root Bridge Or A Secondary Root Bridge - 3Com S7906E Configuration Manual

Two or more MSTP-enabled devices belong to the same MST region only if they are configured to
have the same format selector (0 by default, not configurable), MST region name, the same
VLAN-to-instance mapping entries in the MST region and the same MST region revision level, and
they are interconnected via a physical link.
The configuration of MST region–related parameters, especially the VLAN-to-instance mapping
table, will cause MSTP to launch a new spanning tree calculation process, which may result in
network topology instability. To reduce the possibility of topology instability caused by configuration,
MSTP will not immediately launch a new spanning tree calculation process when processing MST
region–related configurations; instead, such configurations will take effect only after you activate
the MST region–related parameters using the active region-configuration command, or enable
MSTP using the stp enable command in the case that MSTP is not enabled.

Configuring the Root Bridge or a Secondary Root Bridge

MSTP can determine the root bridge of a spanning tree through MSTP calculation. Alternatively, you
can specify the current device as the root bridge or a secondary root bridge using the commands
provided by the system.
Note that:
A device has independent roles in different MSTIs. It can act as the root bridge or a secondary root
bridge of one MSTI while being the root bridge or a secondary root bridge of another MSTI.
However, the same device cannot be the root bridge and a secondary root bridge in the same MSTI
at the same time.
There is only one root bridge in effect in a spanning tree instance. If two or more devices have been
designated to be root bridges of the same spanning tree instance, MSTP will select the device with
the lowest MAC address as the root bridge.
When the root bridge of an instance fails or is shut down, the secondary root bridge (if you have
specified one) can take over the role of the primary root bridge. However, if you specify a new
primary root bridge for the instance then, the secondary root bridge will not become the root bridge.
If you have specified multiple secondary root bridges for an instance, when the root bridge fails,
MSTP will select the secondary root bridge with the lowest MAC address as the new root bridge.
Configuring the current device as the root bridge of a specific spanning tree
Follow these steps to configure the current device as the root bridge of a specific spanning tree:
To do...
Enter system view
Configure the current device as
the root bridge of a specific
spanning tree
Use the command...
system-view
stp [ instance instance-id ]
root primary
1-18
Remarks
—
Required
By default, a device does not
function as the root bridge of
any spanning tree.