I am busy preparing for CCNA exams -640-816 however the content is not very clear regarding Stubby Areas and Not so Stubby Areas, more specifically I am keen to know why we create stubby areas and how does this affect routing between areas.
are stub and nssa a part of the objectives for -816 or are you just looking for clarification...I thought those were ROUTE subjects.
the routing table for a router in a NORMAL OSPF area (area 0 or any other area number) will show several different types of routes:
O - intra area routes (inside my area)
O IA - inter area routers (from another area in our AS)
O E1 or O E2 - Routes imported (redistributed or otherwise) into OSPF from a different AS (a.k.a EXTERNAL routes)
O*xx - Default routes, either from O, O IA, or O Ex
the routing table for a router in a STUB OSPF area will never show the E type routes. The ABR for the stub area changes the E type to an O*, or default route pointing back to itself...so a router inside of a stub area will have O and O IA routes and for any EXTERNAL routes, it will just have a default route back to the ABR
The NSSA (Not-So-Stubby-Area) is a special type of stub area that allows an ASBR to be present INSIDE the NSSA. Autonomous System Border Routers are the boundary between the OSPF AS and an external AS (generally using a different routing protocol...). The ASBR in an NSSA area will generate a TYPE 4 and TYPE 7 LSA. Type 4 identifies the existance of the ASBR and Type 7 provides the route to the external network or networks (probably being redistributed by the ASBR into OSPF).
Inside the NSSA, the routes that the ASBR imports show up as O N1 or O N2 routes (N2 by default)
At the ABR between the NSSA and the backbone, the Type 7 LSAs are converted to Type 5 LSAs, and from there the Type 5 LSAs are passed on to normal and stub areas as O E1 or O E2 routes (E2 by default). The Type 4 LSAs are passed along to the backbone unmodified.
thanks for the detail explanation, I just needed some clarification and also more in depth reading so I know more for the CCNA exam than what they expect