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:

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:

• There should be at least one ABR in the area.
• All routers in the stub area have to be configured as stub router.
• There is no ASBR within the stub or totally stub area.
• The backbone area cannot become stub or totally stub area.

I hope this overview is useful to you. In another lesson I’ll demonstrate how to configure these different stub areas!