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

Discussion in 'Cisco' started by jimbo, Apr 21, 2005.

  1. jimbo

    jimbo Guest

    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
    jimbo, Apr 21, 2005
    #1
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  2. jimbo

    Victor Guest

    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
    Victor, Apr 21, 2005
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  3. jimbo

    jimbo Guest

    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!
    jimbo, Apr 25, 2005
    #3
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