At what point should one begin to segregate OSPF into multiple Areas ?

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Nov 1st, 2007
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We have a network of approximately 5000 end nodes. The backbone consists of the traditional Cisco model with 2 cores, a data centre layer and distribution layer based on location to feed different terminals across the Airport. All backbone devices are Cat 6500's utilising a mixture of sup 720 / sup 2 all with the MSFC duaghter cards.

I am due to start a new project to introduce a resilient switch within each distribution block and thought it worthwhile to address / optimise any routing.

We currently have all OSPF devices configured in Area 0. It isn't causing us any issues, and memory utilisation is fairly low, with approximately 450 routes at the core.

At what point should I consider segregating the distribution switch blocks into different areas and introduce ABR's ?

Bearing in mind that each distribution switch provides connectivity for somewhere between 1500 - 2000 end hosts.

Thanks in advance.

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dominic.caron Thu, 11/01/2007 - 08:29
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It does not depend on the number of host. You must look at the number of route, the number of router in you design and the type of router.

If this is only a LAN and all router are Sup2 of Sup720. With the number of route you have, 1 area will keep it simple.

Now, let's say you want to build a L3 access network in the future. The access switch will have to have the complete database is there is only 1 area. They will also have to re-calculate on every routing event anywhere on the network. The resources of the access layer switch might be over-utilized. In this scenario, many area with good summarization will spare your resources.

In conclusion, it depends on how you plan the future of your network.

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cbeswick Thu, 11/01/2007 - 08:41
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Hi dominic,

Thanks for your response.

Thats not a bad point that. I hadn't considered the effect this may have on access layer switches utilising layer 3. Although we have no immediate plans to migrate to layer 3 at the access layer, making a provision for this may be worthwhile.

andrew.burns Thu, 11/01/2007 - 10:35
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It's worth having a look at Cisco's general recommendations here:

Although the recommendation suggests no more than 50 routers per area I've seen 200 in a single area running with no problems. It does depend on a combination of factors such as network stability, no. of neighbours, no. of routes, etc.




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