The Member VLAN is the VLAN used to transmit data as described earlier.The Control VLAN is used to perform the health checks on the ring. The Control VLAN can always passthrough all ports in the ring, including the secondary port of the Master node.Ring StatusThe ring failure notification and the ring status checks provide two ways to ensure the ring remains upand active in the event of a switch or port failure.Ring CheckingAt specified intervals, the Master node sends a ring health frame (RHF) through the ring. If the ring iscomplete, the frame is received on its secondary port and the Master node resets its fail-period timer andcontinues normal operation.If the Master node does not receive the RHF before the fail-period timer expires (a configurable timer),the Master node moves from the Normal state to the Ring-Fault state and unblocks its Secondary port.The Master node also clears its forwarding table and sends a control frame to all other nodes, instructingthem to also clear their forwarding tables. Immediately after clearing its forwarding table, each nodestarts learning the new topology.Ring FailureIf a Transit node detects a link down on any of its ports on the FRRP ring, it immediately sends a link-down control frame on the Control VLAN to the Master node.When the Master node receives this control frame, the Master node moves from the Normal state to theRing-Fault state and unblocks its Secondary port. The Master node clears its routing table and sends acontrol frame to all other ring nodes, instructing them to clear their routing tables as well. Immediatelyafter clearing its routing table, each node begins learning the new topology.Ring RestorationThe Master node continues sending ring health frames out its primary port even when operating in theRing-Fault state.After the ring is restored, the next status check frame is received on the Master node's Secondary port.This causes the Master node to transition back to the Normal state. The Master node then logically blocksnon-control frames on the Secondary port, clears its own forwarding table, and sends a control frame tothe Transit nodes, instructing them to clear their forwarding tables and re-learn the topology.During the time between the Transit node detecting that its link is restored and the Master nodedetecting that the ring is restored, the Master node’s Secondary port is still forwarding traffic. This cancreate a temporary loop in the topology. To prevent this, the Transit node places all the ring portstransiting the newly restored port into a temporary blocked state. The Transit node remembers whichport has been temporarily blocked and places it into a pre- forwarding state. When the Transit node inthe pre-forwarding state receives the control frame instructing it to clear its routing table, it does so andunblocks the previously blocked ring ports on the newly restored port. Then the Transit node returns tothe Normal state.338 Force10 Resilient Ring Protocol (FRRP)
