OSPF: Is an NSSA required for Internet access just within an area?

Say for example you just want to provide ISP/Internet access to a single area (not area 0), and the router to access the ISP is located within the same area.

In that case, what is the best area type to use to get the network propagated throughout the area, all else being equal? Would a not-so-stubby-area be the ticket?

Thanx! Jimbo

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Hola jimbo

Some nice Theory to help you ...

NSSA External LSAs are originated by ASBRs within not-so-stubby areas (NSSAs). An NSSA External LSA is almost identical to an AS External LSA. Unlike AS External LSAs, which are flooded throughout an OSPF autonomous system, NSSA external LSAs are flooded only within the not-so-stubby area in which it was originated. The command show ip ospf database nssa-externaldisplays NSSA external LSAs

Not-so-stubby areas(NSSAs) allow external routes to be advertised into the OSPF autonomous system while retaining the characteristics of a stub area to the rest of the autonomous system. To do this, the ASBR in an NSSA will originate type 7 LSAs to advertise the external destinations. These NSSA External LSAs are flooded throughout the NSSA but are blocked at the ABR.

The NSSA External LSA has a flag in its header known as the P-bit. The NSSA ASBR has the option of setting or clearing the P-bit. If the NSSA's ABR receives a type 7 LSA with the P-bit set to one, it will translate the type 7 LSA into a type 5 LSA and flood it throughout the other areas. If the P-bit is set to zero, no translation will take place and the destination in the type 7 LSA will not be advertised outside of the NSSA.

!HTH Saludos, Victor

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That is pretty much in keeping with what I was reading, only explained more succinctly. The question remains however; let me rephrase it; Is NSSA the best area type for a "stub" area that has an Internet router attached to the area (and the path to the ISP should remain in the area)? The idea is that just the networks in the stub area need Internet access though the area's ASBR.

I am wondering if the route would propagate throughout the area regardless. You would NOT be enabling OSPF on the interface connected to the ISP. Right? But the address you configure on the interface would enter the route table. Right? So, would the route propagate to to other routers in the area with out other special configuration, or would the area type need to be NSSA, which seems most closley architected to handle it.

Or is an NSSA area really only suited for if you are connecting to another Autonomous system which is advertising networks to you (and of course you want to have the routes learned through the ASBR in the NSSA and not have them propagated to other areas).


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