7.11 Other Stubby Area Types

CCNP RS 300-101, Optimizing OSPF Behavior, Uncategorized

First, a recap about LSAs.

  • Type 1 – Represents a router
  • Type 2 – Represents a DR for a multi-access link
  • Type 3 – Represents a summary for IA routes
  • Type 4 – Represents an ASBR, usually accompanied by Type 5
  • Type 5 – External LSA for routes originating outside the OSPF domain
  • Type 7 – Allows for advertisement of external routes in stub areas that otherwise do not allow for the existence of Type 4 or Type 5 LSAs

… and a recap about Area Types.

  • Backbone (normal) Area – Area 0
  • Standard (normal) Area – An area adjacent to the backbone with no special stub restriction
  • Stub Area – An area that restricts propagation of Type 5 LSAs, replacing them with a default route using Type 3 LSA (but maintains other inter-area Type 3 summaries as well).
  • Totally Stubby Area – An area that restricts Type 3, 4, and 5 LSAs, replacing the summary LSA with a single default route (with default cost of 1, which can be changed). Other inter-area summaries are removed/restricted.
In this graphic, Area 1 is representing both NSSA and TNSSA as depicted by the NSSA and TNSSA labels. Redistribution of EIGRP from ASBR2 uses Type 7 LSAs due to the stub condition that forbids use of Type 4 and Type 5 LSAs. Those Type 7 LSAs are converted to Type 5 LSAs at the ABR when flooding to Area 0 neighbors.
  • Not-so-stubby Area (NSSA) – Similar to a stub area, it restricts Type 5 LSAs and replaces them with a summary from an ABR, but if the NSSA also contains an ASBR that needs to redistribute routes and flood it to other areas of the network, it uses Type 7 LSAs to forward to the upstream ABR, which are then converted to Type 5 LSAs in that standard area.
  • Totally Not-so-stubby Area (TNSSA) – This is an NSSA that operates as a Totally stubby area with the Type 7 caveat; it restricts Type 3 LSAs and injects a default route while providing redistribution using Type 7 LSAs.

Note: An ASBR is NOT permitted in a stub or totally stubby area because of the restriction on Type 4 and Type 5 LSAs. This is when you use an NSSA or TNSSA.

Commands of Note:

Stub Area Configuration. Note all routers in the area must be configured in a similar fashion to properly form adjacency.
Totally Stubby Area Configuration. The no-summary command is used on the ABR to disable forwarding of Type 3 LSAS.
Not-so-Stubby Area Configuration. Note this will allow the ABR to send a Type 3 summary into the NSSA, but does not provide a default route.
Not-so-Stubby Area Configuration including default route from ABR, accomplished using the default-information originate argument.
Totally Not-so-Stubby Area Configuration. ABR sends only default route with no Type 3 LSAs from ABR, but allows Type 7 for flooding external routes to other non-stub areas.
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