In OSPF, you can make an area as stub if it is not the area 0 (backbone area). But why we need to make the area as stub and what are the different types of stub areas?
Before I dig into this chapter, please be sure that you have a very good understanding of the LSA types that I have already discussed in this book on chapter 9.
There are 4 different of stub areas in OSPF:
- Stub area
- Totally stub area
- NSSA (not so stubby area)
- Totally NSSA (Totally not so stubby area)
As being a hard topic, I would like to show it to you in an illustration, so it becomes more easy to be understood.
We use the different type of stub areas so we can keep the LSAs flooding to a minimum, LSDB smaller and less SFP calculation as well as smaller routing table.
Here is the explanation of the different types of OSPF stub areas:
In the above picture, you see that we have 4 areas connected to the backbone area. Let’s imagine that we have planned to make area 1 as a stub area. Once area 1 is a stub area, then it will block all LSA types 5, and surely you will not allow to have an ASBR because the LSA type 5 is blocked. Now the question is how would the router in the stub area reach to other networks? For this, there will be a default route so the router in the stub area can reach other OSPF networks.
We have seen in the stub are that LSA type 5 will be blocked and there would be no possibility for an ASBR. But what if we want to have the area to be stub but we need to have an ASBR, is there any solution?
Here there is an exception, and we have to make the area as NSSA (not-so-stubby-area). This type of stubby area is the same as stub area with one exception that you can have an ASBR within the area. Of course, you may wonder how this is possible because we are blocking LSA type 5? Here comes the type 7 external LSA. As we are not allowed to use the type 5 external LSA, then NSSA areas will use a new LSA type which is type 7 external LSA. In our graph, this happens on area 3.
The 3rd type of stub areas is the Totally Stub Area. In this area, all LSA type 3 and type 5 will be blocked. As LSA type 5 is blocked, then there is no possibility to have an ASBR in this area.
In this area, you will only see a default route in the routing table. On the graph, area 2 is a totally stub area.
The last type of stub areas is the Totally NSSA. On the Totally Stub Area we have seen that LSA type 3 and Type 5 will be blocked and there will be on ASBR in the area. What if we need to have an ASBR in that area while keep having LSA type 3 and type 5 blocked? Here comes the Totally NSSA.
This is all what I wanted to explain about the different types of stub areas that we have in OSPF. Let’s apply now some LABS on this topic so you can understand them better.