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

 
 
jimbo
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      04-21-2005
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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Victor
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      04-21-2005
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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jimbo
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      04-25-2005
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).

Cheers!

 
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