Juniper EX9200 Features Manual page 248

Spanning-Tree Protocols Feature Guide for EX9200 Switches
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232
However, a blocking interface can transition to the forwarding state in error if the interface
stops receiving BPDUs from its designated port on the segment. Such a transition error
can occur when there is a hardware error on the switch or software configuration error
between the switch and its neighbor.
When loop protection is enabled, the spanning-tree topology detects root ports and
blocked ports and makes sure both keep receiving BPDUs. If a loop-protection-enabled
interface stops receiving BPDUs from its designated port, it reacts as it would react to a
problem with the physical connection on this interface. It does not transition the interface
to a forwarding state, but instead transitions it to a loop-inconsistent state. The interface
recovers and then it transitions back to the spanning-tree blocking state as soon as it
receives a BPDU.
We recommend that you enable loop protection on all switch interfaces that have a
chance of becoming root or designated ports. Loop protection is most effective when
enabled in the entire switched network. When you enable loop protection, you must
configure at least one action (
Note that an interface can be configured for either loop protection or root protection, but
not for both.
Example: Configuring Loop Protection to Prevent Interfaces from Transitioning from
Blocking to Forwarding in a Spanning Tree on EX Series Switches
Understanding Root Protection for STP, RSTP, VSTP, and MSTP on EX Series Switches
on page 233
Understanding BPDU Protection for STP, RSTP, and MSTP on EX Series Switches on
page 217
Understanding MSTP for EX Series and QFX Series Switches on page 78
Understanding RSTP for EX Series and QFX Series Switches on page 3
Understanding STP for EX Series Switches
Understanding VSTP for EX Series Switches and QFX Series Switches on page 159
log , block , or both).
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