OSPF NSSAs, Totally NSSAs and Default Routes

When configuring OSPF with NSSA (Not-So-Stubby Area) and Totally NSSA, it is crucial to understand the behavior of default routes within these areas. For a typical OSPF deployment, the NSSA stub area allows for the use of an ASBR (Autonomous System Boundary Router) that can advertise static default routes to the rest of the topology. This setup permits a localized default route without conflicting with any externally advertised default routes through an ABR (Area Border Router).

In contrast, a Totally NSSA is appropriate only if the region should have a single default route advertised by the ABR to the backbone area, while the ASBR within the area may connect to a network that uses specific subnet advertisements instead of a default route. A potential issue occurs when multiple default routes exist, as one will inevitably be preferred based on administrative distance, which determines route installation on the neighboring router.

For a scenario involving local static routes with an NSSA, it’s optimal to use an NSSA configuration, allowing flexibility with static default routing. This prevents any unintended preference over dynamic routes from the ABR. If multiple routes lead to unexpected routing behavior, adjusting administrative distances or reevaluating route design can resolve the issue, ensuring efficient network traffic management.

Links

https://networklessons.com/ospf/introduction-to-ospf-stub-areas

https://networklessons.com/ospf/how-to-configure-ospf-nssa-not-so-stubby-area

https://networklessons.com/ospf/how-to-configure-ospf-totally-nssa-area