OSPF Area Types in Networking

OSPF Area Types

• OSPF Area Types: 5 types, but only 3 types of area types are distinct (Normal, Stub, and NSSA)
• All areas start as a Normal Area, other types are optimizations of Normal Area

Normal Area

• Default area type
• Allows all types of LSAs (Type 1, 2, 3, 4, and 5)
• All routers in a Normal Area maintain all LSAs

Stub Area

• Optimization of Normal Area
• Does not allow Type 4 and Type 5 LSAs
• Injects a default route via a Type 3 LSA
• Reduces the number of LSAs routers need to maintain

Totally Stub Area (Totally Stubby)

• Further optimization of Stub Area
• Does not allow Type 3 LSAs, only the default route
• Reduces the number of LSAs routers need to maintain even more

Not So Stubby Area (NSSA)

• Optimization of Normal Area
• Allows Type 1, 2, and 3 LSAs
• Does not allow Type 4 and 5 LSAs
• Uses Type 7 LSAs for redistribution
• Type 7 LSAs are translated to Type 5 LSAs by the ABR

Totally Not So Stubby Area (Totally NSSA)

• Further optimization of NSSA
• Does not allow Type 3 LSAs, only the default route
• Reduces the number of LSAs routers need to maintain even more

Area Type Flag

• Part of the Hello Packet
• Two flags: N bit (NSSA) and E bit (External Routing)
• Combining N and E bits determines the area type
  • 1 in E bit and 0 in N bit: Normal Area
  • 0 in E bit and 0 in N bit: Stub Area
  • 0 in E bit and 1 in N bit: NSSA Area

OSPF Area Types

• There are 5 types of OSPF areas, but only 3 are distinct: Normal, Stub, and NSSA.
• All areas start as Normal Areas, and other types are optimizations of Normal Areas.

Normal Area

• The default area type that allows all types of LSAs (Type 1, 2, 3, 4, and 5).
• All routers in a Normal Area maintain all LSAs.

Stub Area

• An optimization of Normal Area that does not allow Type 4 and Type 5 LSAs.
• Injects a default route via a Type 3 LSA, reducing the number of LSAs routers need to maintain.

Totally Stub Area (Totally Stubby)

• A further optimization of Stub Area that does not allow Type 3 LSAs, only the default route.
• Reduces the number of LSAs routers need to maintain even more.

Not So Stubby Area (NSSA)

• An optimization of Normal Area that allows Type 1, 2, and 3 LSAs.
• Does not allow Type 4 and 5 LSAs, and uses Type 7 LSAs for redistribution.
• Type 7 LSAs are translated to Type 5 LSAs by the ABR.

Totally Not So Stubby Area (Totally NSSA)

• A further optimization of NSSA that does not allow Type 3 LSAs, only the default route.
• Reduces the number of LSAs routers need to maintain even more.

Area Type Flag

• The Area Type Flag is part of the Hello Packet.
• It consists of two flags: N bit (NSSA) and E bit (External Routing).
• Combining N and E bits determines the area type:
  • N bit 0, E bit 1: Normal Area
  • N bit 0, E bit 0: Stub Area
  • N bit 1, E bit 0: NSSA Area