SONET/SDH Behavior During Unified ISSU

During a unified in-service software upgrade, several aspects of SONET behavior differ from normal operation.

SONET APS is not supported.
During a conventional software upgrade, a SONET loss-of-signal defect lasts more than 2.5 seconds, causing the router to declare an LOS failure. Devices on the remote end of SONET links detect the failure and bring down the link and the dynamic interface stacks built on the link.
During a unified in-service software upgrade, the LOS does not last more than 2.5 seconds. The remote device detect a momentary LOS but does not perceive this short LOS as a link failure and does not bring the link down,