Typical standard Annex B behavior when a local SF is detected on the primary section (circuit), and this SF is the highest priority request on both the local side and from the remote side as per the APS specifications, is to send a request to the remote end and then wait until a reverse request is received before switching over to the secondary section. To reduce the recovery time for traffic, the router switches over to the secondary section immediately upon detecting the local SF on the primary section instead of waiting for the reverse request from the remote side. If the remote request is not received after a period of time then an ‟PSB Failure is declared” event is raised (Protection Switching Byte Failure – indicates an inconsistent or invalid Rx K1 Bytes), and the APS group on the local side switches back to the primary section.
When the remote side is in Lockout, and a local SF is detected then a reverse request is not received by the local side. In this case, the traffic no longer flows on the APS group because neither the primary nor secondary sections can carry traffic, and the outage reduction optimization causes a temporary switchover from the primary to the secondary and then back again (which causes no additional outage or traffic issue because neither section is usable). If this temporary switchover is not wanted then it is recommended to either perform Lockout from the router side, or to Lockout from both sides, which avoids the possibility of the temporary switchover.
Failures detected on the secondary section cause immediate switch over as per the Annex B specification. There is no outage reduction optimization in the router for this case as it is not needed.
Some examples of events that can cause a local SF to be detected include: a cable being cut, laser transmitter or receiver failure, a port administratively ‟shutdown”, MDA failure or shutdown, IOM failure or shutdown.
In Annex B operation, all switch requests are for a switch from the primary section to the secondary section. After a switch request clears normally, traffic is maintained on the section to which it was switched by making that section the primary section. The primary section may be working circuit 1 or working circuit 2 at any particular moment.