What is an NSSA, why would you use one, and how do Type 7 LSAs become Type 5?
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An NSSA is a stub area that is allowed to have an ASBR. The ASBR originates Type 7 LSAs that flood only
inside the NSSA. The NSSA ABR translates them into Type 5 for the rest of the domain. Externals from
elsewhere stay out. Totally NSSA, with no-summary on the ABR, also replaces Type 3 with a default.
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Typical use
A branch area that should stay small, but has a local internet breakout or a partner connection redistributed into OSPF at the branch.
Config
! ABR R2
router ospf 1
area 2 nssa no-summary
!
! ASBR R4 inside area 2
router ospf 1
area 2 nssa
redistribute static subnetssubnets is needed on older IOS releases to redistribute non-classful prefixes. Newer IOS-XE adds it automatically.
Translation
- The ABR translates Type 7 to Type 5 only when the P bit is set, which the ASBR sets by default.
- With several ABRs, the one with the highest router ID is elected translator.
- The translated Type 5 has the ABR as advertising router.
Default routes
- Plain NSSA: no automatic default on IOS. Add
area 2 nssa default-information-originateon the ABR, which creates a Type 7 default (shown asO*N2). - Totally NSSA: the ABR injects a Type 3 default automatically (
O*IA).
They will ask next
An ABR is also the ASBR for the NSSA. How do you stop it from sending its own redistributed routes into the NSSA?
area 2 nssa no-redistribution. The routes still go into area 0 as Type 5, but are not put into the NSSA as Type 7.
Traps
- Saying the NSSA ASBR generates Type 5. It generates Type 7 and the ABR translates.
- Forgetting the missing default in plain NSSA, then wondering why branch internet traffic fails.