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.

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