The master node sends a health-check frame on the Control VLAN at a
user-configurable interval. If the ring is complete, the health-
check frame will be received on its secondary port, where the master
node will reset its fail-period timer and continue normal operation.
If the master node does not receive the health-check frame before the
fail-period timer expires, the master node moves from the normal
state to the "ring-fault" state and unblocks its secondary port. The
master node also flushes its bridging table and sends a control frame
to all other nodes, instructing them to also flush their bridging
tables. Immediately after flushing its bridge table, each node
starts learning the new topology. This ring polling mechanism
provides a backup in the event that the Link Down Alert frame should
get lost for some unforeseen reason.
The master node continues sending periodic health-check frames out
its primary port even when operating in the ring-fault state. Once
the ring is restored, the next health-check frame will be received on
the master node's secondary port. This will cause the master node to
transition back to the normal state, logically block non-control
frames on the secondary port, flush its own bridge table, and send a
control frame to the transit nodes, instructing them to flush their
bridging tables and re-learn the topology.
During the time between the transit node detecting that its link is
restored and the master node detecting that the ring is restored, the
secondary port of the master node is still open -- creating the
possibility of a temporary loop in the topology. To prevent this,
the transit node will place all the protected VLANs transiting the
newly restored port into a temporary blocked state, remember which
port has been temporarily blocked, and transition into the "pre-
forwarding" state. When the transit node in the "pre-forwarding"
state receives a control frame instructing it to flush its bridging
table, it will flush the bridging table, unblock the previously
blocked protected VLANs on the newly restored port, and transition to
the "normal" state.