# Unexpected SONET and SDH Behavior During Unified ISSU

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

- SONET APS is supported only for non-channelized APS IOAs.

  During a unified in-service software upgrade, if you have configured APS functionality on the non-channelized APS IOAs, the unified ISSU process fails and a warning message is displayed. If you have not configured APS functionality, the unified ISSU process succeeds and the line modules (OC3/OC12) do not get rebooted.

  | Note: The unified ISSU process for the channelized APS IOAs has not been modified. The channelized APS IOAs are rebooted during a unified in-service software upgrade.

- 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,

#### Related Documentation

- Application Support for Unified ISSU