LSA delayed acknowledging

When an OSPF device receives LSAs from its neighbor, it acknowledges the LSAs. After the acknowledgement is received, the neighbor removes this device from its retransmission list and stops resending the LSAs.

In the case of NSR, the device fails after receiving the LSA from its neighbor and acknowledges that neighbor upon receipt of an LSA. The LSA synchronization to the standby module is then completed. In this case the standby module, when taking over from the active module, does not have that LSA in its database and the already acknowledged neighbor does not retransmit that LSA. For this reason, the NSR-capable device waits for LSA synchronization of the standby module to complete (Sync-Ack) before acknowledging the neighbor that sent the LSA.