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Forum Replies

  1. andrew says:

    A type-3 default route generated by an ABR into its directly attached totally stubby area is an exception to the no LSA type 3 rule. This is the only type-3 route allowed within an totally stubby area.

  2. Hello Yamini

    First of all, there are two flavours of NSSAs. There are NSSAs that block type 5 and type 4 LSAs but allow type 3 LSAs and there are NSSA totally stubby areas that allow only summary default routes and everything else is filtered.

    In the first case, in order to make a stub area into an NSSA, the following commands are used:

    router ospf 1
     area 1 nssa

    This command must be configured on every router in Area 1. After defining area 1 as an NSSA, tye 5 and type 4 LSAs are blocked, but type 3 is allowed.

    In the case of a totally stubby NSSA, you must issue the following command under the OSPF configuration:

    router ospf 1
     area 1 nssa no-summary

    This command is configured only on the NSSA ABR. After you define the NSSA totally stub area, Area 1 has these characteristics in addition to the NSSA characteristics:

    • No type 3 or 4 summary LSAs are allowed in area 1. This means no inter-area routes are allowed in area 1.
    • A default route is injected into the NSSA totally stub area as a type 3 summary LSA.

    So, concerning the default route, there are two ways to have a default route in an NSSA. When you configure an area as NSSA, by default the NSSA ABR does not generate a default summary route and you require a static default route. In the case of an NSSA totally stubby area, the NSSA ABR does generate a default summary route and no static default route is necessary.

    I hope this has been helpful!


  3. Hello Yamini

    Rene's lesson on types of stub areas very clearly indicates the functionality of each. You can find it here.

    However, I'll try to give you a quick summary of why you would use each case in a real life scenario.

    A stub area would be configured if a network segment had only one choice for routing all traffic. An example of this would be a branch office that has only one gateway. There is no need for LSAs of type 5 to be advertised within this network since all routes will point to the local default gateway.

    A totally stubby area would be configured if you have additional OSPF areas in your network which means that Type 3 LSAs would also be sent among OSPF routers. The totally stubby area would also block Type 3 LSAs since they too would be routed via the one and only gateway available to the network segment.

    A Not So Stubby Area (NSSA) is defined as a stub that includes an ASBR to another routing protocol autonomous system. In this case, you do not require Type 5 LSAs since there is only one route to other OSPF areas. So you would configure an NSSA if you have an additional "way out" of the network, but via an ASBR to an AS of a different routing protocol.

    I hope this has been helpful!


  4. Let me use another topology to explain where/why you could use stub areas:

    OSPF is our “campus” network. Area 0 is the main network, area 1,2, and 3 are branch offices. BGP routes are redistributed into OSPF, RIP routes are also redistributed into OSPF.

    With regular areas, all routers will learn about all prefixes out there. When you look at this picture…why would area 1, 2, or 3 need to know the prefixes from BGP? There is only one path, and that is through area 0. We can make area 1,2 and 3 stub areas…a default route is more than enough.

    Also, let’s say a router in area 1 wants to go to a router in area 2. Do we need to know a specific prefix? Not really, there is only one path through area 0…in other words, why not make area 1,2, and 3 totally stub areas…get rid of all inter-area routes and just use a default route.

    That does introduce one problem…a router in area 3 is doing redistribution from RIP into OSPF, that’s something we can’t do in a stub or totally stub area. That’s why you should convert area 3 into a totally NSSA…it’s a totally stub area that does allow an ASBR.

    Btw, as soon as you configure a router to redistribute something into OSPF, it’s an ASBR.

    Hope this helps.

  5. ah don’t do that Rene you already are very responsive and take care of the website. Save your time for important stuff it was small inconvenience only I just glossed over the names and focused on specific granular that I was having problems with but makes me feel bad if you try to do something that difficult in the time consuming meticulous area.

    I think its fine. I saw in another post once where someone griped about named routers and maybe some truth but then you said you might change it then you went back and fixed it. That had repercussion of making some of the forums post be off no biggie you was just being too helpful lol! I would not worry about changing all past things unless its something that is easy and efficient just use new method going forward.

    You are to important to all of us to be spending your time trying to read through things you have already worked on in the past and only a limited number of the hardcore study people will read through.

    Your website is already the best out there for content, and for team that is responsive to replying and helping. I am a professional student as I love to learn I would not pull your leg or inflate your ego.

    This is hands down the best networking site with content and forums. Not many people with your knowledge and ability would do what you do to the level you do it. Oh and lets give credit to those moderators as well like Las and Andrew. Great team!

    Hands down the most cost efficient way to learn about Cisco Networking this site is worth double or triple what you charge but I am thankful you give the people such a great deal. I am a happy person by nature but I am also a very honest person and I am not a person that believes in being inefficient and wasting my time talking up their company because I like them or was just being nice. No that’s all truth.

    I think we will all live =)

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