Signaling board isolation occurs when the inter-board link fails. In this case, neither board can communicate with the other and cannot distinguish this case from a failure of its mate board.
During isolation, the primary board keeps running but at (potentially) reduced capacity since the signaling links on the backup board cannot be accessed. Normal checkpointing of ISUP circuit states to the backup board from the host can still take place.
When isolation is detected on the backup board, the active signaling links are put into an isolated state, queuing inbound packets but still delivering any queued outbound packets, and a short isolation timer is set. If the isolation ceases before the timer expires, normal traffic is resumed starting with the queued packets. If the isolation timer expires before the isolation condition is corrected, the isolated links are placed into the local processor outage (LPO) state and the queued inbound packets are discarded.
Switching the backup board into primary mode (as would be the case if the primary board failed) clears the isolated/LPO condition on those links and resumes normal traffic flow.
After the failed inter-board link is restored, the active MTP 3 layer clears the LPO condition on the isolated links to restore normal traffic and checkpoints any route or link states that may have changed during the isolation.
There is no isolation timer or processor outage-like state associated with SIGTRAN. No action is taken upon an isolation event other than to log a message and change the isolatedState status field.