OSPF Area Types-

  • Stub
  • Totally Stubby
  • NSSA (Not-so-stubby areas)
  • Totally NSSAs

Let’s cover stub and totally stubby with the below lab topology.

 

Stub Area – No Type 5 LSA

Let’s talk about Stub, just by glancing at the topology we can tell that R5 is a stub router and area-2 is a stub area, which means that OSPF stub areas prohibit Type 5 LSAs (External Routes) and Type 4 LSAs (ASBR summary) from entering the Stub area.

Let’s look at the routing table for R4 and R5 without any stub command.

We can see the external routes in the routing table for both R4 and R5.

The goal is to reduce the size of the routing table and not see the external routes. All routers in the stub area must be configured as stubs to form an adjacency.
Let’s run the stub command on R4 and R5 and see the routing table.

R4, R5
Router ospf 1
Area 2 stub

Now lets take a look at the routing tables for R4 and R5 after converting to stub.

We’re not seeing a big change in the routing table for R4 even after converting to stub, this is not a mistake. At this point R4 will generate a default route when it receives external routes to R5.

Let’s look at R5’s routing table.

We can clearly see that the *IA default route generated by R4, even though R5 does not see the 10.0.0.0/24 (External EIGRP network) subnet in it’s routing table, it does have connectivity to that network through the default route.

We can ping from R5 to verify.

Totally Stubby- No LSA Type 3, 4, 5

Totally stubby area prohibits LSA type 3, type 4 and type 5 from entering the totally stubby area. The only routes we will see in the Totally stubby area will be the OSPF routes within area-3.

Let’s look at the routing table for R4 and R6 before converting to totally stubby.

As we see the routing tables are similar, now we will convert area 3 into totally stubby.

 

R4, R6
Router ospf 1
Area 3 stub no-summary

 

The routing table for R4 will not change, because it’s generating the default route but let’s take a look at R6’s routing table.