OSPF LSA types

The special area types were created in order to filter certain LSA types; the trick here is to know the LSAs that are going to be filtered, because the routes are represented by the allowed LSAs inside the area. Let's define the "must know" LSAs at a basic level:

 A network diagram showing different OSPF areas and the LSAs found in each area.  Note, STUB area is similar to NSSA and Totally STUB is similar to Totally NSSA.  The main difference is the NSSA and Totally NSSA include an ASBR and Type 7 LSAs. 

Type-1 LSA – It’s is the Router LSA; this is defined in order to send the link state information about the attributes of the directly connected routes, besides of that you have just ONE type-1 LSA per every router inside your area.

Type-2 LSA – It’s the Network LSA; this LSA is specifically defined for Broadcast and Non-Broadcast network types. This is managed by the DR of the segment and there is just ONE type-2 LSA per segment.

Type-3 LSA - This is the Summary-net LSA; this LSA is normally in designs where you have multiple areas, so the ABR is in charged to convert the intra-area LSA types to this type of LSA in order to differentiate the intra-area routes from the inter-are routes. There is one type-3 LSA per route.

Type-4 LSA - This is the ASBR Summary LSA this basically identifies the ASBR of a specific area (note that inside the area where the ASBR is this LSA will not exist, instead the type-1 LSA will be used in order to announce the ASBR of the area). Who is the ASBR of the area? The type-4 LSA is there in order to answer this question, so this is injected by the ABR in order to announce the presence of the ASBR in a particular area.

LSA-type 5 – This is the ASBR External LSA; these basically are the external routes, one type-5 LSA per route.

LSA-type 7 - This is a special LSA used for special area types like NSSA and Totally-Stubby NSSA, this basically resides in a particular area and doesn't purely go out instead is translated by the ABR to type-5 LSA. This logic is very important because this is the foundation to learn how the NSSA and Totally Stubby NSSA special areas work.

Now that you have general view of the LSAs and what they do you will see that the general objectives of the special area types is to filter the LSAs that are entering from the area border router (ABR) to the area, and in the case of the NSSA areas is to filter the LSAs from the ABR, but passing the external routes inside the area to the other areas. You will normally find these areas where there is only one exit point (but in some cases you can have designs where multiple exit points exist), that is just one ABR in order to go out of the area.

There are basically four types of special areas:

Stub Areas - This is RFC compliant. This basically filters the LSA-type 4 and 5 in order to avoid the reception of external routes inbound the area, note that the type 5 LSA are basically the external routes plus their attributes, and the type 4 LSAs are the attributes of the ASBR. You will just have inter-area routes inside this area (routes with the "IO" code) in the routing table. This behavior is accomplished with the "area x stub" command inside the OSPF global process, note that ALL the routers inside the stub area will need to set the stub flag to establish the adjacencies.

Totally Stubby Area - This is Cisco proprietary. The Totally Stubby area is there to filter type-3, type-4, and type-5 LSAs, so what are you going to receive in order to reach the inter-area, and external routes? You ABR will help your area injecting a "quad zero" default-route inside the area. The command to accomplish this is the "area x stub no-summary" inside the OSPF global process.

Not-So-Stubby-Area (NSSA) - This area is there for situations where you are redistributing external routes, BUT you want to filter the LSAs that could come from your ABR. This areas use a "tricky" mechanism in order to do this, which is the type-7 LSA. The type-7 LSA is there in order to trick the area, because normally in this types of areas you have external routes (from external autonomous systems), so you will need to send those routes through your ABR in order to inject them into the other areas, BUT as i said filtering the reception of the type-4 and type-5 LSAs. You will basically see "IO" routes (they are in the routing table because of the type-3 LSAs), besides the "N1/2" routes which represents the NSSA area. Note that in order to inject the already mentioned default route you will have hardcode it. Note that this is  not the case of the other special areas. The "area x nssa default-information originate" is used in order to accomplish this mentioned behavior (if you don't add the "default-information originate" keyword the router will l not be able to reach unknown destinations, because the default route will not be injected.

Totally-Stubby NSSA – It’s Cisco Proprietary. The exact same dilemma of the NSSA in the sense of redistribution inside the area, but wanting to filter not only type-4 and 5 LSAs, but type-3 LSAs, so inside the area you will only have a default route, besides the "N1/2" routes that are normal on that type of area. The "area x nssa no-summary" is used in order to make the area TS-NSSA.

If an area is converted to a special area ALL routers must agree about the stub flag, so ALL the routers must have the “area x <stub–flag on>, because if this is not the case the adjacencies will not be established.