Questions and Answers
What is the primary function of a Type 7 LSA in an NSSA area?
Redistribute external routes into the OSPF domain
Which of the following area types does not allow Type 3 LSAs?
Totally Not So Stubby Area
What is the purpose of the N bit in the Area Type Flag?
Identifies an NSSA area
Which of the following statements is true about Normal areas?
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What is the purpose of the E bit in the Area Type Flag?
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Which of the following area types is an optimization of a Normal area?
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What is the main difference between a Stub area and a Totally Stub area?
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What is the result of combining the E bit and N bit in the Area Type Flag?
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Study Notes
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
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