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New Member

OSPF Path Selection Issue

Hello Guys,

I am in a little trouble here, 2 questions ! please give away your comments:

scenario:

In a multisite network enviornment, SITE-A is having multi-service providers and making GRE tunnels with Site-B & Site-C.

Site-A is running backbone Area 0, while all boundry routers are acting as ABR for Area-4 & Area-3 via GRE Tunnels

Problem:

Route-Site-A-2 is the lowest cost to reach Site-B , On Core router at Site-A you'll see Router-Site-A-2 and Site-B as a selected path....but when you go to any ABR... you'll see its own Tunnel as the best exit path via OSPF, although other ABR's are recieving shortest path via OSPF neighbours.

In OSPF RFC i read:

> the order of preference for OSPF routes is:

> intra-area routes, O

> interarea routes, O IA

> external routes type 1, O E1

> external routes type 2, O E2

so how do this issue justifies with another opinion by a cisco's expert that

"As being in a different area, OSPF will opt for the nearest exit - It pays no attention to the metric on the link...."

Another question is: Area-3 doesn't wantes to recieve routes from Area-4 and vice versa but connection to Area 0, is distributing the routes/flooding to both these areas.

I have tried making both Area 3 & 4 a stub but it's not helping...next thing i tried was to drop the routes coming active by filter-lists in OSPF but obviously it's not stopping the OSPF database packets...what can be the possible solution..

thanks in advance...

Shakeel

2 REPLIES
New Member

Re: OSPF Path Selection Issue

URL for Network Deisgn Picture: http://70.84.169.162/~shakeel/OSPF.jpg

Re: OSPF Path Selection Issue

Hi,

You are correct about the OSPF order of preference. But, do you see the OSPF routes being prefered contrary to the RFC 2328. If so, can you be more specific about the problem. Can you provide a show ip route for the network that you think the order of preference is violated.

For the 2nd part of the question, you need to configure site B & C as totally stub areas as stub doesn't suppress IA routes. Stub only suppresses external (Type 5 LSA) routes. Totally stub, on the other hand, suppresses IA (type 3 LSA) routes and injects just a default route into the totally stub area.

Hope this helps!

Sundar

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