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Trusted Contributor
Posts: 447
Registered: ‎11-11-2008
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Multiple OSPF instances - One big Area?

Hi All,

 

I have a setup where I have two fairly large Area 0 networks which need to be interconnected.  The address space is unique between the two.  I see two options in this case

 

A)  Make it one large backbone AREA 0. 

 

B) Create a routing instance on the switch that interconnects the two AREA 0.

 

In point A everything will work fine but I feel that SPF calculation in one AREA 0 does not need to really be flooded out to the other Area.

 

In point B I would need to redistribute all the routes from one instance to the other in order to ensure full redundant route reachablitliy.  This is because the two different areas are interconnected redundantly at two different Points  If I have to do this is there still an advantage to splitting up teh two Areas (Basicly creating to OSPF Autonomous systems)

 

 

I attached a diagram to show how this would be interconnected

 

Thanks All!

Super Contributor
Posts: 216
Registered: ‎05-06-2010
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Re: Multiple OSPF instances - One big Area?

While I'm not sure I fully understand all the implications of plan B, my networking philosophy is that simpler is better.  Unless the combined area 0 is absolutely huge I don't think the SFP calculations are going to present a  problem.  Introducing routing instances and route redistribution can really cause troublehsooting headaches down the road.  So my personal preference would be A.

 

I doubt it's an option since you don't list it, but C might be ideal, which would be a complete OSPF area redesign.

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Trusted Contributor
Posts: 447
Registered: ‎11-11-2008
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Re: Multiple OSPF instances - One big Area?

Thanks for the reply B2.  I will be going with Plan A.

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