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Sunday, January 12, 2014

What is the use by making R2 and R5 as Stub ?


In the above OSPF topolgy Router 4 redistribute EIGRP routes in to ospf, when we configure r2 and r5(area 2) as stub external type-5 lSA has been blocked and i have single route in database, so         56          0x80000001 0x0075C0, so i try router 2 become totally- stub area. now its blocked both type 3 and type 5 ISA R5, this time  had only one single default route in router 5 *0 IA. 

 1) What is the use by making those area stub or totally sub ?

Now we configure rip network on Router 5 and try to redistribute n to ospf, it's show this error
 %OSPF-4-ASBR_WITHOUT_VALID_AREA: Router is currently an ASBR while having only one area which is a stub area.

2) Is this error indicates that can't have asbr on stub area  (because of type 5 ISA blocked already) .

Now R5 and R2 running as NSSA and R5 redistribute rip routes in to ospf.everything works great except one,it is not possible to  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 we found that need to add default-information-originate command on Rotuer2 (ABR).

3) Why is it after adding the comment, it's able to ping R4 eigrp router ?

1) 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 sub-optimal 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.

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

3) 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>]

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