It may be necessary to remove a primary board from service to upgrade the software or hardware on the signaling node or the board itself. The recommended procedure is to manually switch the backup board into primary mode before shutting down the (now backup) board or node, as described here.
Once the applications have agreed that a switchover is necessary, the primary board is set into backup mode with hmiBackup. hmiBackup sets all signaling links into a flow-controlled state, resulting in all inbound packets being queued. Each layer (starting from MTP 3 or SIGTRAN on up) then sends a status indication (NOW BACKUP) to each of its service users.
Due to queuing between layers and within the device driver, the application can still receive some incoming signaling traffic between the issuing of the hmiBackup request and receipt of the NOW BACKUP status indication.
For ISUP messages, the following procedure is recommended:
Connect Confirm (answer) and Release Confirm: Accept and checkpoint new circuit state to mate application/ISUP layer.
All others: Discard and allow far end to timeout and retry if desired.
For TCAP, TUP, or SCCP messages, the following procedure is recommended:
For TUP Connect Confirm (answer) and Release Confirm: Accept and checkpoint new circuit state to mate application.
The application must ignore the event and wait for the other end to retry or (especially if the event is a checkpointed TCAP transaction or an SCCP connectionless) reply on the now primary board.
During this period, the application must not generate any new outbound signaling traffic.
Once the now backup status indication is received (indicating the end of any in-progress signaling traffic) the mate board is set to primary mode. This restarts the flow of signaling traffic to/from the mate board/node, including any messages queued within layer 2 during the switchover.
Note: Packets can be lost during a switchover. Heavy traffic during a switchover can result in either or both boards becoming congested due to the queuing of incoming packets. Planned switchovers are not recommended during periods of heavy load. Schedule maintenance during off-peak periods whenever possible.