From CCIE Study Wiki

OSPF areas can be further defined as Stubs in order to reduce utilization of network and system resources.

Theory

Within areas defined as stubs, ABRs stop advertising type 5 LSAs (and sometimes type 3 LSAs) into the area and instead create and advertise a default route. Internal area routers then simply use the default route to forward packets to the ABR, and they have much smaller LSDBs.

OSPF also allows for Not-So-Stubby areas, which means that routers within the stubby area can redistribute external routes into OSPF. The routers will create type 7 LSAs for these routes.

There are four different types of OSPF stub networks:

• Stub - stops type 5 LSAs, doesn't stop type 3 LSAs. No creation of Type 7 LSAs allowed
• Totally Stubby - stops both type 5 and type 3 LSAs, No creation of Type 7 LSAs allowed
• Not-So-Stubby - stops type 5s, doesn't stop type 3, allows external routes to be redistributed in as type 7 LSAs
• Totally Not-So-Stubby, stops both type 5 and 3 LSAs, allows external routes to be redistributed in as type 7 LSAs

Commands

• area area-id stub (ospf config) - defines area as a normal stub network
• area area-id stub no-summary (ospf config) - defines an area as a totally stubby network
• area area-id nssa (ospf config) - defines area as a not-so-stubby network
• area area-id nssa no-summary (ospf config) - defines a totally not-so-stubby area

Default Settings

• Nothing listed yet

Verification

• Nothing listed yet

Troubleshooting, Tips, and Tricks

• Nothing listed yet

Online Resources

• Cisco Learning Module presentation on configuring special OSPF areas (must be registered to view).

• Nice Cisco technote describing NSSA areas and type 7 LSA's.

• Anthony of Internetwork Expert has a series of articles describing OSPF areas:
  • OSPF areas part 1 - The backbone.
  • OSPF areas part 2 - normal and stub areas.
  • OSPF areas part 3 - The totally stubby area.
  • OSPF areas part 4 - The not-so-stubby area.
  • OSPF areas part 5 - The totally not so stubby area.

• A fantastic article detailing OSPF Area and LSA types from Packetlife.net.

• In-depth article from Roy of Route Leaker discussing the OSPF NSSA forward address selection process.

• A summary of the different OSPF areas from Routing Bits.

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