In the context of OSPF (Open Shortest Path First), stub areas and NSSA (Not-So-Stubby Area) are two types of areas that have specific characteristics and purposes within the OSPF routing domain. Let’s explore the key differences between stub areas and NSSA:
Stub Area:
1. Routing Information:
- Limited Routing Information: In a stub area, routers within the area receive a reduced set of routing information compared to standard OSPF areas. Specifically, they do not receive external routes (routes from other Autonomous Systems).
2. ABR (Area Border Router) Role:
- ABR Filtering: The ABR connected to the stub area filters out external routes before advertising information into the stub area. This simplifies the routing table within the stub area.
3. LSAs (Link State Advertisements):
- Type-3 LSAs: Stub areas contain Type-3 LSAs (Summary LSAs) but lack Type-5 LSAs (External LSAs).
4. Routing to External Networks:
- Default Route: Instead of external routes, routers within a stub area are typically configured to use a default route to reach external networks.
NSSA (Not-So-Stubby Area):
1. Routing Information:
- Limited Routing Information: Similar to stub areas, NSSA also limits the routing information within the area, excluding Type-5 LSAs (External LSAs).
2. ABR Role:
- ABR with Special Handling: The ABR connected to the NSSA performs special handling of external routes. Instead of blocking them entirely, it converts Type-7 LSAs (ASBR Summary LSAs) to Type-5 LSAs at the ABR.
3. LSAs:
- Type-7 LSAs: NSSAs contain Type-7 LSAs originating from ASBRs (Autonomous System Boundary Routers) within the NSSA.
4. Routing to External Networks:
- Default Route or Specific Routes: Similar to stub areas, NSSAs often use a default route to reach external networks. Alternatively, specific routes can be configured.
Common Aspects:
1. Routing Table Simplification:
- Both stub areas and NSSAs aim to simplify the routing tables within the OSPF domain by limiting the amount of external routing information.
2. Default Route Usage:
- Both stub areas and NSSAs commonly use a default route to reach external networks, promoting a more concise routing table.
3. Scalability:
- Both area types contribute to the scalability of OSPF by reducing the amount of routing information propagated throughout the network.
Summary:
In summary, while both stub areas and NSSAs limit external routing information and contribute to the simplification of routing tables, NSSAs have a unique characteristic of allowing special handling of external routes through the use of Type-7 LSAs and the conversion to Type-5 LSAs at the ABR.