OSPF Control Plane Looping and p-bit

Hello team,

I was going through OSPF lectures and had some doubts:
Question 1:
How does OSPF avoid control plane looping? I did find an answer, however, couldn’t understand it clearly.
To be specific it is point number 2 and 3. The question is how? Is there any specific bit, flag?

Answer :

Area Border Router (ABR) is a router that has at least one interface in Area 0 and this interface is not in a down state. ABRs will set the B (border) bit in their router LSAs to signal other routers in the same area of their ABR status. Only ABRs are allowed to generate summary LSAs and inject them in the attached areas.
ABRs expect summary LSAs from Area 0 only. This means there should be at least one adjacency in a FULL state built over an Area 0 interface. If an ABR has such an adjacency, it will ignore summary-LSAs received over non-backbone areas. These LSAs will be installed in the database, but will not be used for SPF calculations.
ABRs will accept and use summary-LSAs learned over non-backbone area they do not have a FULL adjacency built over an Area 0 interface. It is safe to do so, since the ABR will not be able to flood the summary back into Area 0 creating routing loops.
Question 2:
Why p-bit is unset when NSSA ASBR is itself a NSSA ABR? Like how would the process be let’s say if we have two NSSA ABR and I redistribute loopback 1 ( into one of the NSSA ABR. It will start acting as NSSA ASBR now. Correct? So will LSA 7 be generated by that particular ABR/ASBR or not? I am totally confused about the scenario.

Thank you in advance.

Jugal Modi

Hello Jugal

Take a look at this response…

I hope this has been helpful!