When external routing information is imported into an NSSA, LSA Type 7 is generated by
the ASBR and it is flooded within that area only. To further distribute the external
information, type 7 LSA is translated into type 5 LSA at the NSSA border. The P-bit in LSA
Type 7 field indicates whether the type 7 LSA should be translated. This P-bit is
automatically set by the NSSA ABR (also the Forwarding Address (FA) is copied from Type
7 LSA). The P-bit is not set only when the NSSA ASBR and NSSA ABR are the same
router for the area. If bit P = 0, then the NSSA ABR must not translate this LSA into Type 5.
The nssa-only keyword instructs the device to instigate Type-7 LSA with cleared P-bit,
thereby, preventing LSA translation to Type 5 on NSSA ABR device.
Note. If a router is attached to another AS and is also an NSSA ABR, it may originate a
both a type-5 and a type-7 LSA for the same network. The type-5 LSA will be flooded to the
backbone and the type-7 will be flooded into the NSSA. If this is the case, the P-bit must be
reset (P=0) in the type-7 LSA so the type-7 LSA isn’t again translated into a type-5 LSA by
another NSSA ABR.
