Introduction to OSPF Stub Areas

OSPF has special area types called stub areas. In this lesson I want to give you an overview of the different OSPF stub areas. Make sure you understand the different OSPF LSA types before you continue reading or it might be difficult to follow me.

Let me start by summing up the special area types:

  • Stub area
  • Totally stub area
  • NSSA (not so stubby area)
  • Totally NSSA (totally not so stubby area)

Do you feel stubby now?

These special area types are used to insert default routes into an area and replace type 3 summary LSAs and type 5 external LSAs. This will keep the LSA flooding to a minimum, LSDB smaller, less SPF calculations and a smaller routing table. Let me give you an overview with the different stub areas:

OSPF Stub Areas

Let’s start with a nice topology with 5 areas. In the middle you’ll find the backbone area and the other areas are configured as the different stub area types.

If you configure an area as stub it will block all type 5 external LSAs. All the prefixes that you redistributed into OSPF from another routing protocol are not welcome in the stub area. Since you are not allowed to have type 5 external LSAs in the stub area it’s also impossible to have an ASBR in the stub area. In order to reach networks in other areas there will be a default route.

Of course there’s always an exception. So what if you want an area to be stub area but you also have an ASBR in this area? You can use the NSSA (not-so-stubby-area). This is the same thing as the stub area with the exception that you are allowed to have an ASBR within the area. How does it work? This is where the type 7 external LSA kicks in. Since we are not allowed to use the type 5 external LSA we’ll just use a new LSA type.

What about totally stub? This area type will block type 5 external LSAs and type 3 summary LSAs. It’s impossible to have an ASBR in the totally stub area since type 5 external LSAs are blocked.

If you want to block type 3 summary LSAs and type 5 external LSAs but still need an ASBR within the totally stub area you can turn it into a totally NSSA (totally not-so-stubby-area).  This will block both LSA types but you can still have an ASBR in this area type.

Anything else you need to know? Here are some of the rules when dealing with the stub and totally stub areas:

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Forum Replies

  1. It has to do with the design of the network in question. The benefits you see of an area becoming a stub (reduced LSDB size) comes at a cost, which is the loss of some routing information details. This translates into routers within a stub not having all the information necessary to make the best possible choices.

    For example, suppose you have an area (which is non-zero, of course), that has multiple exit points. Now imagine that at each of those exit points there are separate external routing domains (say, EIGRP or BGP, etc). If this area is a stub, Type-5

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  2. Thanks for making a video on this series. I am always thankful when I get to a video series as I read the book and website pages and just get worn out from studying 2-6 hours a day through the week. Not to mention on CCNP ROUTE I have taken to reading every single forum post as an added learning tool.

    Sometimes I just want to lean back in chair put on headset and listen to video as it allows me to relax a bit when tired towards end of day so uch thanks for those videos. Anyway great lesson!

    I am getting close to end of OSPF website lessons already finished

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  3. HI,
    I have a question on this point.
    Doesnt ABR just generate type3 LSA or type 4 to let us know of ASBR.
    Now in TSA, the only allowed LSA’s are 1 & 2. So how does ABR help ?
    if an ABR is configured with default originate always, my understanding is that route alwas appears as an O *E2 or Type 5 or External route in the downlink routers . But I am seeing it as O *IA in my setup, R1 – R2(ABR) – R3(TSA) , pls let me know why ?
    Addtionally in show ip ospf dbs cmd on TSA router , I see only LSA1 and Sumary Net link States (LSA3) being shown, should not LSA 3 be b

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  4. Hello Sahil

    An ABR will generate type 3 LSAs in order to inform other areas of the routes that are found in a particular area. Type 3 LSAs are sometimes known as a summary LSA, that summarises all networks within an area. A type 4 LSA is used to inform other areas of the existence of an ASBR.

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  5. Hello Boris

    The question is not completely clear, as it does not clarify what it means by “prevent type 4 and type 5”. Does it mean that those LSA types should not exist within the stub area? If so, then all of them should be correct. Take a look at the following table:

    However, if we take the question to mean “prevent ONLY type 4 and type 5 LSAs and not Types 1, 2 and 3”, then the answer would be NSSA area and Stubby area as @fugazz suggested. Th

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