At startup, the backup and primary applications must open the ISUP or TUP service with a call to ctaOpenServices:
Note: Circuits default to a flow-controlled state. Until an MTPRESUME indication is received indicating that circuits are available, requests referencing these circuits return an error indication with cause 27: no route to destination.
Upon startup, the backup application receives a batch checkpoint from the primary application. As shown in the following illustration, the backup application checkpoints this information to the ISUP stack running on the backup board:
