Signal Fail (SF) and SF recovery provide the mechanism to repair the ring to preserve connectivity among customer networks.
ERP guarantees that although physically the topology is a ring, logically it is loop-free. One link, called the Ring Protection Link (RPL), is blocked to traffic. When a non-RPL link fails in the ring, the SF mechanism triggers and causes the RPL to become forwarding. Later, signal fail recovery can occur to restore the ring to the original setup.
Convergence time is the total time that it takes for the RPL owner to receive the R-APS (NR) message and block the RPL port until the ERN with the failed link receives notice and unblocks the failed link.
The following figure shows a simple Ethernet ring topology before a failure. This diagram shows dual-end blocking enabled (thick line) between ERNs one (RPL node) and 6 (RPL owner). ERNs 3, 2, 4, and 5 are non-RPL nodes.
The following figure shows the same Ethernet ring topology after a failure at the forwarding port of ERN 4 when a signal fail triggered, and ring protection was needed. ERN 6 unblocked the RPL port and the RPL node changed the blocking port to the forwarding state.
