Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.100.0 - Page 316

Ring Status The ring failure notification and the ring status checks provide two ways to ensure the ring remains up and active in the event of a switch or port failure. Ring Checking At specified intervals, the Master node sends a ring health frame (RHF) through the ring. If the ring is complete, the frame is received on its secondary port and the Master node resets its fail-period timer and continues 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, instructing them to also clear their forwarding tables. Immediately after clearing its forwarding table, each node starts learning the new topology. Ring Failure If 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 the Ring-Fault state and unblocks its Secondary port. The Master node clears its routing table and sends a control frame to all other ring nodes, instructing them to clear their routing tables as well. Immediately after clearing its routing table, each node begins learning the new topology. Ring Restoration The Master node continues sending ring health frames out its primary port even when operating in the Ring-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 blocks non-control frames on the Secondary port, clears its own forwarding table, and sends a control frame to the 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 node detecting that the ring is restored, the Master node's Secondary port is still forwarding traffic. This can create a temporary loop in the topology. To prevent this, the Transit node places all the ring ports transiting the newly restored port into a temporary blocked state. The Transit node remembers which port has been temporarily blocked and places it into a pre- forwarding state. When the Transit node in the pre-forwarding state receives the control frame instructing it to clear its routing table, it does so and unblocks the previously blocked ring ports on the newly restored port. Then the Transit node returns to the Normal state. Multiple FRRP Rings Up to 255 rings are allowed per system and multiple rings can be run on one system. More than the recommended number of rings may cause interface instability. You can configure multiple rings with a single switch connection; a single ring can have multiple FRRP groups; multiple rings can be connected with a common link. The platform supports up to 32 rings on a system (including stacked units). Member VLAN Spanning Two Rings Connected by One Switch A member VLAN can span two rings interconnected by a common switch, in a figure-eight style topology. A switch can act as a Master node for one FRRP group and a Transit for another FRRP group, or it can be a Transit node for both rings. 316 Force10 Resilient Ring Protocol (FRRP)