Fast Stack Link Failover, or Fast Link Failover, is a process designed to reduce the duration of data packet loss when one of the stack ports link fails. This process is only supported on a ring topology since a stack can still remain operational even if a link in the topology fails.
The objective of this document is to understand the Fast Link Failover process on the SG550XG and SG350XG Series Managed Switches.
For a full length demonstration of Fast Link Failover, please view the video below:
Suppose we have 4 units in a stack, resulting in a ring topology. Unit 1 can therefore send traffic to unit 2 or unit 4. If unit 1 is supposed to send traffic to unit 3, the traffic can be routed in two ways: unit 1 to unit 2 to unit 3, or unit 1 to unit 4 to unit 3.
Therefore, if the link between units 1 and 2 fails, unit 1 can send traffic to unit 3 via unit 4, and vice versa. Until the stack topology is recovered, the stack unit loops back the packets that are supposed to be sent through its failed stacking port, and transmits the looped back packets through its remaining stacking port to their destination.
Fast Link Failover is a feature that allows minimizing data packet loss when one of the stack ports link fails. Until the stack topology is recovered, the packets are looped back through the remaining stacking port to the destination, resulting in a fast link failover.