In this lesson, I’ll explore the different types of stub areas in OSPF and their configurations on Juniper routers. Stub areas help simplify routing tables by reducing the number of routes that a router in a specific area must maintain. This is particularly useful in OSPF multi-area configurations where certain areas do not need full routing information.

Types of Stub Areas

There are four types of stub areas in OSPF:

1. Stub Area

Definition: A standard stub area blocks Type 5 LSAs, preventing external routes from entering the area. The routers in the stub area receive a default route from the Area Border Router (ABR) to reach external networks.

Key Points:

• External routes (Type 5 LSAs) are blocked.
• A default route is injected by the ABR for external connectivity.

2. Totally Stubby Area

Definition: A Totally Stubby Area blocks both Type 5 LSAs and inter-area routes (Type 3 LSAs). Routers in this area only receive a default route from the ABR for all external and inter-area destinations.

Key Points:

• Blocks both Type 5 (external) and Type 3 (inter-area) LSAs.
• Only a default route is injected for all destinations.

3. Not-So-Stubby Area (NSSA)

Definition: An NSSA allows for the injection of external routes (Type 7 LSAs) within the area, but these LSAs are not propagated beyond the NSSA. Instead, they are translated into Type 5 LSAs by the ABR when leaving the NSSA.

Key Points:

• Allows external routes within the NSSA (Type 7 LSAs).
• Type 7 LSAs are translated into Type 5 LSAs by the ABR.

4. Totally NSSA

Definition: A Totally NSSA blocks both Type 5 and Type 3 LSAs, similar to a Totally Stubby Area. However, it allows Type 7 LSAs within the NSSA.

Key Points:

• Blocks Type 5 LSAs and Type 3 LSAs (inter-area routes).
• Allows Type 7 LSAs for external routes within the NSSA.

Configuration Notes

To configure a stub area in Juniper routers:

• Configure the stub settings on all routers in the area, including the ABR.
• Ensure that Area 0 is never configured as a stub area.
• Use the appropriate keywords like stub, no-summary, or nssa depending on the type of stub area.

 

Lab Scenario

 

I have now to apply the theory that we have learned about the stub areas. I will apply it in a lab and show you the four different stub areas to see what would be the result once we configure an area as a stub or any of the other types of stub areas. As you can see here, we have a lab of four points. Before I start doing those points, as usual, I’ll go back to the scenario to show you what we need to do and then come back to the points to start working on them.

This is my scenario. I’m still on the same lab so OSPF is running, everything is fine here, and the virtual link is configured. Everything we’ve done so far in this course is still in place. What I want to do now is make Area 2 a stub area. I’ll configure it as a stub, totally stubby, NSSA, and totally NSSA. For this, I must configure the routers (R3 and R4) that connect Area 2. Once the area type is set, I’ll check the routing table on R4 to verify the results.

Configuration Steps

Point 1: Make Area 2 a Stub Area and Check the Routes

First, remember that when an area is a stub area:

• Type 5 (external LSAs) will not be present.
• No ASBR (Autonomous System Boundary Router) is allowed in the area.

Since Area 2 does not have an ASBR, it meets the stub area requirements. Let’s configure Area 2 as a stub area and verify the changes.

As seen in the output above, the external LSA (Type 5) and the default route are visible in R4’s OSPF database. Once Area 2 is configured as a stub area, these external LSAs should disappear.

Step 1: Configure Stub Area

Now that Area 2 is configured as a stub, let’s check R4’s OSPF database again to verify the absence of Type 5 LSAs.

The output shows that Type 5 LSAs are no longer present in the OSPF database. This confirms that Area 2 is now a stub area.

Point 2: Make Area 2 a Totally Stubby Area

In a Totally Stubby Area, not only are Type 5 LSAs excluded, but also inter-area routes (Type 3 LSAs) are replaced with a default route.

Let’s verify the routing table on R4:

The routing table now shows only a default route (0.0.0.0/0) and no inter-area routes. This confirms that Area 2 is now a Totally Stubby Area.

 

Point 3: Configure Area 2 as NSSA

In an NSSA (Not So Stubby Area), Type 5 LSAs are replaced with Type 7 LSAs to allow for external routes.

Check R4’s OSPF database for Type 7 LSAs:

The output confirms the presence of Type 7 LSAs in Area 2.

 

Point 4: Configure Area 2 as Totally NSSA

In a Totally NSSA, Type 5 LSAs are replaced with Type 7 LSAs, and inter-area routes (Type 3 LSAs) are replaced with a default route.

Verify the routing table on R4:

The routing table now includes a default route and Type 7 LSAs. This confirms that Area 2 is configured as a Totally NSSA.

Conclusion

In this lab, we have successfully configured the four types of stub areas (Stub, Totally Stubby, NSSA, Totally NSSA) and verified the resulting OSPF databases and routing tables. Understanding these configurations helps in optimizing OSPF networks by controlling the flow of LSAs.