H3C Technologies H3C S10500 Series Switches - Manual (Page 90)

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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 connected 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.
• The device in PVST mode automatically maps VLANs to MSTIs, and supports more MSTIs than in MSTP
mode. When you change the spanning tree mode from PVST to MSTP, exceeding VLAN-to-instance
mappings (arranged in ascending order of MSTI IDs) are silently deleted and cannot be recovered even
if you change the spanning tree mode back. To prevent loss of mappings, do not manually configure
VLAN-to-instance mappings in PVST mode.
Configuring the root bridge or a secondary root bridge
You can have MSTP determine the root bridge of a spanning tree through MSTP calculation, or you can
specify the current device as the root bridge or as a secondary root bridge using the commands that the
system provides.
A device 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, one device 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 devices are designated as the root bridge
in a spanning tree at the same time, the device with the lowest MAC address wins.
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 one you specify, not the secondary root bridge will 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 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... Use the command... Remarks
Enter system view system-view —
Configure the current device as the
root bridge (in STP/RSTP mode)
stp root primary
Configure the current device as the
root bridge (in PVST mode)
stp vlan vlan-list root primary
Configure the current device as the
root bridge (in MSTP mode) stp [ instance instance-id ] root
primary
Required
Use any command.
By default, a device does not
function as the root bridge.
Configuring the current device as a secondary root bridge of a specific spanning tree
Follow these steps to configure the current device as a secondary root bridge of a specific spanning tree: