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learning and the priority vector is worse than that already held by the port. If a disputed BPDU is
received, the port is forced to the listening state. When an inferior designated BPDU with the
learning bit set is received on a designated port, its state is set to discarding to prevent loop
formation. Note that the Dispute mechanism is always active regardless of the configuration
setting of Loop Protection.
Loop Protect operates as a per port, per MST instance feature. It should be set on inter‐switch
links. It is comprised of several related functions:
•
Control of port forwarding state based on reception of agreement BPDUs
•
Control of port forwarding state based on reception of disputed BPDUs
•
Communicating port non‐forwarding status through traps and syslog messages
•
Disabling a port based on frequency of failure events
Port forwarding state in the designated port is gated by a timer that is set upon BPDU reception. It
is analogous to the rcvdInfoWhile timer the port uses when receiving root information in the root/
alternate/backup role.
There are two operational modes for Loop Protect on a port. If the port is connected to a device
known to implement Loop Protect, it uses full functional mode. Otherwise the port operates in
limited functional mode.
Connection to a Loop Protect switch guarantees that the alternate agreement mechanism is
implemented. This means the designated port can rely on receiving a response to its proposal
regardless of the role of the connected port, which has two important implications. First, the
designated port connected to a non‐root port may transition to forwarding. Second, there is no
ambiguity when a timeout happens; a Loop Protect event has occurred.
In full functional mode, when a type 2 BPDU is received and the port is designated and point‐to‐
point, the timer is set to 3 times helloTime. In limited functional mode there is the additional
requirement that the flags field indicate a root role. If the port is a boundary port the MSTIs for
that port follow the CIST, that is, the MSTI port timers are set according to the CIST port timer. If
the port is internal to the region then the MSTI port timers are set independently using the
particular MSTI message.
Message age expiration and the expiration of the Loop Protect timer are both Loop Protect events.
A notice level syslog message is produced for each such event. Traps may be configured to report
these events as well. A syslog message and trap may be configured for disputed BPDUs.
It is also configurable to force the locking of a SID/port for the occurrence of one or more events.
When the configured number of events happen within a given window of time, the port is forced
into blocking and held there until it is manually unlocked via management.
Configuring Spanning Tree Bridge Parameters
Purpose
To display and set Spanning Tree bridge parameters, including device priorities, hello time,
maximum wait time, forward delay, path cost, and topology change trap suppression.
Configuring Spanning Tree Bridge Parameters
D-Series CLI Reference 6-3
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