OSPF NSSA

[feroz syed]

Hi,

I have this ospf topology,Router 4 redistribute eigrp routes in to ospf, when i configure r2 and r5(area 2) as stub external type-5 lsa has been blocked and i have single route in database, so 0.0.0.0         2.2.2.2         56          0x80000001 0x0075C0, so i try router 2 become totally- stub area. now its blocked both type 3 and type 5 lsa n R5, this time i had only one single default route in router 5 *0 IA.  1) what is the use by making those area stub or totally sub ? just making route table and database small or any thing else ??

Now i configure rip network on Router 5 and try to redistribute n to ospf, it's show this error

*Dec 26 00:48:05.016: %OSPF-4-ASBR_WITHOUT_VALID_AREA: Router is currently an ASBR while having only one area which is a stub area.

This error indicate that can't have asbr on stub area  (bcoz of type 5 lsa blocked already) am right??.

Now R5 and R2 running as NSSA and R5 redistribute rip routes in to ospf.everything works great except one, i can't reach R4 eigrp network,Router 4,3 and 1 can able to pinging R5 rip network, and Router 2 (ABR) can ping R4 eigrp network.But i found answer on google that need to add default-information-originate command on Rotuer2 (ABR). Can anybody explain me why i should added that cmd and after add how it's able to ping R4 eigrp routes.

Any help would be very much appreciated 

[Rolf Fischer · Accepted solution]

Hi,

1) what is the use by making those area stub or totally sub ? just making route table and database small or any thing else ??

Stub Areas save resources in terms of CPU, memory and network traffic by filtering out certain information and replacing them by a default-route. A potential drawback is loss of detail information if more than one exit-points exist, which could cause some suboptimal paths. In your topology there is only one exit-point, so it doesn't matter if you have 1000 exteral routes, all pointing to the same ABR, or simply one default-route, providing the very same information: The outside world is reachable through the ARB (R2) - just like leaving a dead-end street.

This error indicate that can't have asbr on stub area  (bcoz of type 5 lsa blocked already) am right??.

Correct. The concept of a NSSA was introduced to meet such requirements.

Can anybody explain me why i should added that cmd and after add how it's able to ping R4 eigrp routes.

Unlike Stub Area Broder Routers, a NSSA Border Router does not inject a default route automatically, you have to configure it manually if needed (similar to Regular Areas):

area nssa default-information-originate [metric <0-16777214>] [metric-type <1-2>]

HTH

Rolf