# Unexpected T3 Behavior During Unified ISSU

Local T3 (DS3) devices are reprogrammed during a unified in-service software upgrade, generating a defect. The router completes the reprogramming within 2.5 seconds. Because JunosE DS3 applications declare an alarm and bring down the link only if the defect persists for more than 2.5 seconds, unified ISSU does not cause the links to be brought down. However, the remote T3 devices must also wait 2.5 seconds before declaring an alarm. If the equipment on the far end of the T3 connection generates an alarm immediately rather than waiting, the link goes down, causing the higher layers to also go down for the remote equipment.

#### Related Documentation

- Application Support for Unified ISSU