OSPF default routes

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OSPF has numerous, sometimes confusing and contradicting default routing options. The default routes OSPF routes can generate include:

  • External default route, advertised as type-5 LSA into the OSPF domain. This default route is not propagated to stub or NSSA areas.
  • Stub area default route, which is used by routers in a stub are to reach external destinations. ABR routers always advertise a default route into a stub area and IOS allows you to change its cost.
  • NSSA area default route. IOS allows you to generate type-7 (NSSA external) or type-3 (inter-area) default route into a NSSA area.

On top of all these options, IOS supports conditional OSPF default route origination.

Read more in the OSPF Default Mysteries IP corner article

Conditional OSPF default route origination based on classless IP prefixes

Recent IOS releases, including IOS release 12.4, 15.0 and 12.2SR, support conditional OSPF default route origination based on route maps using IP prefix lists. The IP prefix-list used in the route map can match any IP prefix as long as the match is exact (the le and ge keywords are not allowed).

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External default route in NSSA area

OSPF NSSA area accepts external routes redistributed into the OSPF domain as NSSA external (LSA type 7) routes that are later translated by an ABR into domain-wide external (LSA type 5) routes. In designs where the default exit from the OSPF domain (for example, Internet connection) resides in an NSSA area, a router in an NSSA area has to insert a default route into the NSSA area (a sample network is displayed in Figure 1).

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