352-001 · Question #146
What are two functions of an NSSA in an OSPF network design? (Choose two.)
The correct answer is B. It allows ASBRs to inject external routing information into the area. C. An ASBR advertises Type 7 LSAs into the area. An OSPF NSSA allows external routes to be injected into a stub-like area via an ASBR using Type 7 LSAs, which the ABR then translates to Type 5 LSAs for redistribution.
Question
What are two functions of an NSSA in an OSPF network design? (Choose two.)
Options
- AIt overcomes issues with suboptimal routing when there are multiple exit points from the area.
- BIt allows ASBRs to inject external routing information into the area.
- CAn ASBR advertises Type 7 LSAs into the area.
- DAn ABR advertises Type 7 LSAs into the area.
- EIt uses opaque LSAs.
How the community answered
(19 responses)- A5% (1)
- B95% (18)
Why each option
An OSPF NSSA allows external routes to be injected into a stub-like area via an ASBR using Type 7 LSAs, which the ABR then translates to Type 5 LSAs for redistribution.
Addressing suboptimal routing with multiple exit points is not a function of NSSA; that concern is typically addressed with OSPF cost tuning or specific forwarding address behavior.
An NSSA permits an ASBR to exist within the area and redistribute external routing information, which is otherwise prohibited in a standard stub area. This is the primary reason NSSAs are used - to allow controlled external route injection without accepting all external LSAs.
Within an NSSA, the ASBR generates Type 7 LSAs to carry external routing information inside the area. These Type 7 LSAs are specific to NSSAs and are translated to Type 5 LSAs by the ABR when they leave the area.
The ABR does not advertise Type 7 LSAs into the area - the ASBR originates Type 7 LSAs; the ABR translates Type 7 LSAs to Type 5 LSAs at the area boundary for propagation outside the NSSA.
Opaque LSAs (Types 9, 10, 11) are used for OSPF extensions such as traffic engineering and MPLS, not as a feature or mechanism of NSSA operation.
Concept tested: OSPF NSSA Type 7 LSA function and ASBR role
Source: https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/6208-nssa.html
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