Hello Ibrahim,
the P bit when not set means that an LSA type 7 should not be propagated into the OSPF backbone.
It is checked by the ABR with highest OSPF RID to decide if the LSA type 7 has to be translated to type 5 or not.
So the P bit allows to build a scenario like the following:
An ASBR inside an NSSA area can generate an external route as a LSA type 7 that will remain confined within the NSSA area.
The possible use case is the need to connect to an external domain that is using IP prefixes that might be in conflict with other OSPF areas and other route types.
So instead of using NAT the routes can be kept local to the NSSA area.
To be noted in wireshark capture you can see the NP bit:
where N stands for NSSA area when set in OSPF Hello flags or Router LSA.
P is used in LSA type 7 as explained above , but in the capture the bit is named NP in both cases.
Hope to help
Giuseppe
