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OSPF NSSA stub area type - not clear on why it exists

Jun 14, 2010 10:30 AM

bitlock76 17 posts since
Mar 11, 2010

I'm not clear on why OSPF has the NSSA stub area type.  From the looks of it, in both the text and my test lab, it is the same as a non-stub area, except that external networks are flooded with a type 7 LSA rather than a type 5.  Does anyone have any insight as to what conditions would make this is helpful, or if there's something else that happens that I'm not aware of?

  • dkarpekin 124 posts since
    Jan 21, 2009

    With no stub, if you do "show ip route ospf" it will gave all routes to other area. By definition NSSA http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a88.shtml    it is can be "middle man area", making similar to default gateway functionality.

    Therefore once you enable "area 1stub", "area 1 stub no-summary" it will summarize routes and simplify routing table, still yet allowing you options for "fine-tuning"- which route can be present or not in NSSA routing table.

    Simply to put it will do close to same result as  default route in static routing command.

    See attach, where STUB was used to summiraze route in GNS3. There is some comments,I put for myslef as reminder.

    Attachments:
  • Claudius 117 posts since
    Jan 7, 2009

    Hi Bitlock

     

    I understand your thinking about NSSA, It seems to be the same unless you have an ASBR into the NSSA area, In a nutshell, the point of this Area type is to have the improvements given by the Stub feature while having the ability to get external routes thru the ASBR into the NSSA and therefore in the OSPF domain, that otherwise will be not allowed. So in fact is a feature that gives you more design flexibilty while keeping the OSPF routing information concise thanks to the stub feature.

     

     

    HTH,

     

    Carlos

  • Brian 2,971 posts since
    Aug 17, 2009

    Aloha Bitlock76,

     

    You are correct, in that STUB and NSSA are very similar.  Likewise Totally STUB and Totally NSSA are very similar.  The main difference is that NSSA and Totally NSSA areas allow the OSPF domain to connect to external networks via an ASBR.  STUB and Totally STUB areas only connect back to Area 0.  Go to the "documents" tab under the CCNP Study Group and there are a couple of documents that may help you understand the different OSPF areas and the LSA types for each.

     

    HTH

  • VivianFlorencia 174 posts since
    Sep 29, 2009
    Currently Being Moderated
    4. Jun 14, 2010 1:13 PM (in response to Brian)
    Re: OSPF NSSA stub area type - not clear on why it exists

    Hola Brian,

     

    Totally NSSA?

     

    VivianFlorencia.

  • Brian 2,971 posts since
    Aug 17, 2009

    Aloha Vivian,

     

    I take it you never heard of Totally NSSA?  I first heard this term back in 2004 when studying for my CCNP.  The correct term is NSSA Totally Stub, but I like the shortened version, Totally NSSA.  This keeps things similar to the STUB and Totally STUB terminology already in use.  What is the difference between a STUB and Totally STUB?  The Totally STUB includes the "no summary" keywords on the OSPF command.  Example,

     

    STUB

    router ospf 100

    area 1 stub

     

    Totally STUB

    rouiter ospf 100

    area 1 stub no summary

     

    Likewise the difference between NSSA and Totally NSSA (or NSSA Totally STUB if you prefer) is that Totally NSSA includes the "no summary" keyword.

     

    NSSA

    router ospf 100

    area 2 nssa

     

    Totally NSSA (or NSSA Totally Stub)

    router ospf 100

    area 2 nssa no summary

     

    Please read the attached Cisco PDF on OSPF NSSA. You can even Google Totally NSSA and read several posts.  Cisco even uses the term on their CCIE R&S v3.0 Written Blue Print.

     

    IP Routing
    A.OSPF
    1.Standard OSPF area
    2.Stub area
    3.Totally stub area
    4.Not-so-stubby-area (NSSA)
    5.Totally NSSA
    6.Link State Advertisement (LSA) types
    7.Adjacency on a point-to-point and on a multi-access (broadcast)
    8.OSPF graceful restart
    9.Troubleshooting failing adjacency formation to fail
    10.Troubleshooting of external route installation in the RIB

     

    The two terms are equal or interchangable, so if you hear it in the future from someone else, you will know what they "really" mean. 

     

    HTH

    Attachments:
  • Brian 2,971 posts since
    Aug 17, 2009

    Aloha bitlock76,

     

    Yes, you are on the right track.  Here is what Cisco says about NSSA areas:

     

    "Not-so-stubby areas (NSSAs) are an extension of OSPF stub areas. Like stub areas, they prevent the
    flooding of AS-external link-state advertisements (LSAs) into NSSAs, relying instead on default
    routing to external destinations. As a result, NSSAs (like stub areas) must be placed at the edge of an
    OSPF routing domain. NSSAs are more flexible than stub areas in that an NSSA can import external
    routes into the OSPF routing domain,
    thereby providing transit service to small routing domains that
    are not part of the OSPF routing domain."

     

    HTH

  • VivianFlorencia 174 posts since
    Sep 29, 2009
    Currently Being Moderated
    8. Jun 15, 2010 9:02 AM (in response to Brian)
    Re: OSPF NSSA stub area type - not clear on why it exists

    Thanks Bri

     

    VivianFlorencia

  • Brian 2,971 posts since
    Aug 17, 2009

    Aloha bitlock76,

     

    In response to your post#9, and referring to your basic diagram, you have the following:

     

    External AS (eigrp) <--> OSPF Area 1 <--> OSPF Area 0 <--> External AS (eigrp)

     

    This means you have an ASBR in both Area 0 and Area 1, which is ok.  However, Area 1 should be configured as NSSA.  By default NSSA does not inject a default route into Area 1.  Area 1 will only see LSA type 1,2,3,and 7.  The LSA type 7 will be changed to LSA type 5 at the ABR between Area 1 and Area 0.  This is exactly the behavior you should see.  Please refer to the attached OSPF Area and LSA diagram.  This explains the different OSPF areas and the LSA allowed in each area.  To get a default route into Area 1 you will have to use one of three configurations.

     

    router ospf 100

    area 1 nssa no summary

     

    OR

     

    router ospf 100

    area 1 nssa default-information originate

     

    OR

    router ospf 100

    area 1 nssa default-information originate always

     

    The last one will inject a default route 0.0.0.0 even when no default route exists in the routing table.  Here are a couple of other documents regarding OSPF default routes.

     

    How OSPF generates default routes

    http://www.ciscosystems.com/application/pdf/paws/13692/21.pdf

     

    How OSPF injects default route in to NSSA

    http://www.cisco.com/application/pdf/paws/47870/ospfdb11.pdf

     

    How OSPF injects default route in to Normal Area

    http://www.cisco.com/application/pdf/paws/47868/ospfdb9.pdf

     

    Check these out and hopefully you find what your looking for.

     

    HTH

    Attachments:
  • Brian 2,971 posts since
    Aug 17, 2009

    Aloha bitlock76,

     

    In response to your post #10, the point of the NSSA is to allow the injection of external routes from an external AS (eigrp in you case).  The NSSA area was created just for this purpose.  With the original STUB area you cannot redistribute routes that were external, it was not allowed.  Hence, we have NSSA, which allows the redistribution of external routes from outside the OSPF domain.  Here is another Cisco PDF on OSPF NSSA.

     

    http://www.cisco.com/application/pdf/paws/6208/nssa.pdf

     

    HTH

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