Notes from studying and discussion of OSPF Areas for the JNCIE-SP and JNCIP-SP exams.
Stub Areas
Stub - Do not permit OSPF External routes (Type 5 LSAs) into the area. Re-generate OSPF Internal routes as Summary Type 3 LSAs.
Stub w/ default metric XXX - Same as above, but advertise a Type 3 Summary default-route into the area with metric XXX to reach external routes.
Stub, no-summaries, default metric XXX (aka Totally Stubby Area) - Do not permit OSPF internal or external routes into the area. Instead, advertise a Type 3 Summary default-route with metric XXX.
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Not-So-Stubby-Areas
NSSA - Fundamentally similar to Stub Area (see above), but ASBR can reside within the area and generates an NSSA External Type 7 LSA for each route exported into OSPF. Type 7s are regenerated as Type 5s into the backbone area by the ABR.
NSSA w/ default-lsa default-metric XXX - Same as above, but advertise a Type 3 Summary default-route into the area with metric XXX to reach external routes.
NSSA, no-summaries, default metric XXX - Do not permit OSPF internal or external routes into the area. Instead, advertise a Type 3 Summary default-route with metric XXX. ASBR can reside within the area…
Optional configuration for NSSA: -
default-lsa {
default-metric XXX;
metric-type YYY;
type-7;
}
NSSA default-routes can be generated as Type 7 for backward compatibility. This affords you the opportunity to set the external metric-type as “1” if you want one to be preferred when available.
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References