What is is use cases of OSPF P Bits , we we need it ???

[Ibrahim Jamil]

Hello Guys What is is use cases of OSPF P Bits , we we need it ??? thanks

[Giuseppe Larosa]

Hello Ibrahim,

in an NSSA area an ASBR router  can inject external routes as LSA type 7 either O N1 or O N2.

The P bit allows the ASBR to signal that the ABR router between (0, NSSA-area) should translate the LSA type 7 to an LSA type 5 to be sent in the backbone.

Note in case of multiple ABR nodes (0, NSSA-area) only the ABR with highest router-id performs the conversion. Forwarding address is set to the ASBR. This is normal and minimizes external LSA in the OSPF domain.

If the P bit is not set the LSA type 7 is confined within the NSSA area and it is not propagated to the rest of OSPF domain.

In wireshark packet captures  this bit is called NP bit, as it used in Router LSA as N bit to indicate NSSA area flag and as P bit in LSA type  7.

Hope to help

Giuseppe

[anthonykahwati]

So I’d like to ask a little deeper on this one. At what point is the p-bit set? I know that it is set to 0 if the ASBR is also the ABR and instead the ABR / ASBR will generate both type 5 and type 7 LSA’s, but what if the ASBR is further away from the ABR? Is the P-bit set at origination / redistribution, is it set by all routers in the area, is it set only by the ABR?