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NOTE:
•
Two or more spanning tree 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, MST region revision level,
and the same VLAN-to-instance mapping entries in the MST region, and they are interconnected via a
physical link.
•
The configuration of MST region–related parameters, especially the VLAN-to-instance mapping table,
will result in a new spanning tree calculation. To reduce the possibility of topology instability, the MST
region configuration takes effect only after you activate it by using the
active region-configuration
command, or enable a spanning tree protocol by using the
stp enable
command in the case that the
spanning tree protocol is disabled.
Configuring the root bridge or a secondary root bridge
The root bridge of a spanning tree is determined through spanning tree calculation. Alternatively, you
can specify the router as the root bridge or a secondary root bridge.
A router has independent roles in different spanning trees. It can act as the root bridge in one spanning
tree and as a secondary root bridge in another. However, a router cannot be the root bridge and a
secondary root bridge in the same spanning tree.
A spanning tree can have one root bridge only. If two or more routers are selected as the root bridge in
a spanning tree at the same time, the router with the lowest MAC address wins out.
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, the secondary root
bridge with the lowest MAC address is selected as the new root bridge.
Configuring the current router as the root bridge of a specific spanning tree
To configure the current router as the root bridge of a specific spanning tree:
Step Command
Remarks
1.
Enter system view.
system-view
N/A
2.
Configure the current router as
the root bridge.
•
In STP/RSTP mode:
stp root primary
•
In MSTP mode:
stp
[
instance
instance-id
]
root
primary
Use one of the commands.
By default, a router does not
function as the root bridge.
Configuring the current router as a secondary root bridge of a specific spanning tree
To configure the current router as a secondary root bridge of a specific spanning tree:
Step Command
Remarks
1.
Enter system view.
system-view
N/A
