234 CHAPTER 23: MSTP CONFIGURATION
To prevent this situation from happening, MSTP provides the root guard function
to protect the root bridge. If the root guard function is enabled on a port, this port
will keep playing the role of designated port on all MST instances. Once this port
receives a configuration BPDU with a higher priority from an MST instance, it
immediately sets that instance port to the listening state, without forwarding the
packet (this is equivalent to disconnecting the link connected with this port). If the
port receives no BPDUs with a higher priority within twice the forwarding delay,
the port will revert to its original state.
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It is recommended that you enable the root guard feature on your device.
Follow these steps to enable root guard:
Enabling Loop Guard By keeping receiving BPDUs from the upstream device, a device can maintain the
state of the root port and other blocked ports. However, due to link congestion or
unidirectional link failures, these ports may fail to receive BPDUs from the
upstream device. In this case, the downstream device will reselect the port roles:
those ports failed to receive upstream BPDUs will become designated ports and
the blocked ports will transition to the forwarding state, resulting in loops in the
switched network. The loop guard function can suppress the occurrence of such
loops.
If a loop guard-enabled port fails to receive BPDUs from the upstream device, and
if the port took part in STP calculation, all the instances on the port, no matter
what roles they play, will be set to, and stay in, the Discarding state.
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It is recommended that you enable the loop guard feature on your device.
Follow these steps to enable loop guard:
To do… Use the command… Remarks
Enter system view system-view -
Enter Ethernet
interface view
or port group
view
Enter Ethernet
interface view
interface interface-type
interface-number
Required
Use either command.
Configurations made in
Ethernet interface view will
take effect on the current
port only; configurations
made in port group view will
take effect on all ports in the
port group.
Enter port
group view
port-group { manual
port-group-name |
aggregation agg-id }
Enable the root guard function
on the port(s)
stp root-protection Required
Disabled by default
To do… Use the command… Remarks
Enter system view system-view -
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