3

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in this diagram we have same ABR and ASBR,this router functions as an ABR between area 1 and zero,plus it works as an ASBR and redistribute external routes from some other routing domain to OSPF domain, so the question is:

  1. why do we need to have an NSSA in here? I mean this router has got one interface in Area 0 and one in Area 1,and one in some other routing domain,it does not belong to a particular area(does not have all its interfaces in one single,individual area)so waht's the reason to have a NSSA in here?and if we are supposed to have NSSA which area will be that?

refereed to this document http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/6208-nssa.html in which talks about "Bit P" and it says "This happens when NSSA ASBR is also an NSSA ABR." so why on the earth in this case do we need to have NSSA? it's so confusing.

thank you

3

You don't need area 1 to be NSSA if it is not a stub area. In that case, the external routes are flooded into both area 1 and 0. But if you want area 1 to be stubby, meaning that it doesn't receive any external LSAs from other areas, then it must be NSSA, because it is injecting external routes.

The external routes are flooded in both areas. But if area 1 is a stub, the routers don't forward type 5 LSAs The only external routes that can be flooded in a stub area are type 7, which by definition makes it a NSSA area.

The reason the P bit is not set is because the router is already injecting type 5 LSAs into area 0, so there's no need for translating type 7 to type 5.

By the way, area 0 is never a stub or nssa.

  • there are 2 vague point in your answer that i cant understand: 1.u said " But if you want area 1 to be stubby, meaning that it doesn't receive any external LSAs from other areas, then it must be NSSA, because it is injecting external routes." why area 1 cant be stubby? i mean if I config area 1 to be stubby,just LSA type 5 will be blocked from entering that,ok?and routers in area 1 will have a default route facing our ABR/ASBR and it must work,right? 2.will our ABR/ASBR in this diagram just inject LSA type 7 to area 1,since it has one int in area 1? – Mjaryan May 17 '14 at 19:40
  • 1. If area 1 is a stub area, then the routers in that area will not flood type 5 LSAs, so no one will know about your injected route. 2. The router will inject both T7 into area 1 and T5 into area 0. – Ron Trunk May 17 '14 at 20:46
  • 1. in this scenario area 1 is stub,it's not transit,so yep u r right but 2. if we config area 1 as stub,so our ABR/ASBR will inject just T7 LSA into the area1 since it's defined as stub and there can't be L5 LSA in stub area?am I right?is my understanding correct? – Mjaryan May 17 '14 at 21:05
  • It will only inject T7 if you make it a NSSA. That's the only place T7 is allowed. – Ron Trunk May 17 '14 at 22:41
1

P-bit is important for the external path selection, as per RFC 3101 :

2.5 Calculating Type-7 AS External Routes

  1. A Type-7 LSA with the P-bit set.
  2. A Type-5 LSA.
  3. The LSA with the higher router ID.

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