Stubby Not-So-Stubby

mattsthe2

Can someone explain to me what a stub area and stubby area and a Not So Stubby area is and does?

dtlokee

The key to understanding stub, totally stub, and no so stubby comes down to knowing your LSA types.

Type 1 - Router LSA, used to announce all the links on a router
Type 2- Network LSA, used to describe a multi-access media like ethernet, including all connected routers
Type 3 - Summary LSA - used by an ABR to send LSA's between areas
Type 5 - External LSA - generated by the ASBR when redistribution into OSPF is taking place (could be from another routing protocol, static, or connected)
Type 7 - Used in an NSSA area to describe external routes, only found in an NSSA area

Ok so a regular area will allow all LSA types throughout the area (except type 7)

A stub area will not allow external (type 5) LSA's in. It relies on a default route to exit the area

A totally stubby area will not allow Type 5 or Type 3 (inter area routes) into the area, again it relies on a static route to exit the area.

A no so stubby area is a special case where you have an area that also needs an ASBR in the area, presumably to allow redistribution from a device that does not support OSPF (a switch with an IP services image perhaps). It is seen as a stub area, it does not allow type 5 LSAs into the area. All the routes redistributed into the nssa area are seen as type 7, which will be turned into type 5 LSAs when the abr sends them from the nssa area to a regular area.

mattsthe2

well explained thanks... would i need this for the CCNA though?

dtlokee

Not really, but it never hurts to know a little more than the average bear