Port Modes and Event Triggers

Ports work in two Loop Protect operational modes. If the port is configured so that it is connected to a switching device known to implement Loop Protect, it uses full functional (enhanced) mode. Otherwise, it operates in limited functional (standard) mode.

Connection to a Loop Protect switching device guarantees that the alternate agreement mechanism is implemented and, therefore, the designated port can rely on receiving a response to its proposal regardless of the role of the connected port. This 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 mode, when a type 2 BPDU is received and the port is designated and point-to-point, the timer is set to 3 times hello time. Limited mode adds a further requirement that the flags field in the BPDU indicates a root role. If the port is a boundary port, the MSTIs for that port follow the CIST (for example if the MSTI port timers are set according to the CIST port timer). If the port is internal to the region, the MSTI port timers are set independently using the particular MSTI message.

Loop Protect initializes the MSTI timer to zero and does not allow the designated port to transition from listening to learning until the timer becomes non-zero. If the port is not designated, the timer does not apply. Its state is controlled through normal protocol behavior.

A disputed BPDU is one in which the flags field indicates a designated role, a learning state, 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.

Message age expiration and the expiration of the Loop Protect timer are both events for which Loop Protect generates a notice level syslog message. You can also configure traps to report these events, as well as a syslog message and trap for disputed BPDUs.

In addition, you can configure Loop Protect to force the locking of an SID/port when one or more events occur. When the configured number of events happen within a given window of time, the port will be forced into blocking and held there until you manually unlock it.