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As a result, loops occur in the switched network. The loop guard feature can suppress the
occurrence of such loops.
The initial state of a loop guard-enabled port is discarding in every MSTI. When the port receives
BPDUs, it transits its state. Otherwise, it stays in the discarding state to prevent temporary loops.
Restrictions and guidelines
Do not enable loop guard on a port that connects user terminals. Otherwise, the port stays in the
discarding state in all MSTIs because it cannot receive BPDUs.
On a port, the loop guard feature is mutually exclusive with the root guard feature or the edge port
setting.
A loop guard-enabled interface can receive BPDUs and transit from the discarding state to the
forwarding state after two forward delays if one of the following events occurs:
• The state of the interface changes from down to up.
• The spanning tree feature is enabled on the up interface.
Procedure
1. Enter system view.
system-view
2. Enter interface view.
interface interface-type interface-number
3. Enable the loop guard feature.
stp loop-protection
By default, loop guard is disabled.
Configuring port role restriction
About port role restriction
Make this configuration on the port that connects to the user access network.
The bridge ID change of a device in the user access network might cause a change to the spanning
tree topology in the core network. To avoid this problem, you can enable port role restriction on a port.
With this feature enabled, when the port receives a superior BPDU, it becomes an alternate port
rather than a root port.
Restrictions and guidelines
Use this feature with caution, because enabling port role restriction on a port might affect the
connectivity of the spanning tree topology.
Procedure
1. Enter system view.
system-view
2. Enter interface view.
interface interface-type interface-number
3. Enable port role restriction.
stp role-restriction
By default, port role restriction is disabled.
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