At what point do you need multiple Areas with OSPF?

I have a pretty small network with under 300 people  - six offices - some very small - in the US.  I have read about OSPF setup and design - how Area 0 needs to touch every other area (with exceptions noted), the different LSA types etc.  But it was never clear to me when you really need to move beyond having just Area 0.  For a small network - is it ok to just use area 0 for say a central site and a few spoke sites?
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amigan_99Network EngineerAsked:
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Don JohnstonConnect With a Mentor InstructorCommented:
Depends on a few of factors. Number of networks, number of routers, RAM installed, to name a few.

Six offices, probably a dozen networks, 2800 and 3800 series routers... I seriously doubt you need to create multiple areas at this time. I'd say you're fine until your network doubles in size.
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Don JohnstonInstructorCommented:
Sure.

There are a couple primary reasons for multiple areas. One is resources. On a big network, the link state database can become huge. This database takes up memory. And running the SPF algorithm on a large database can take time. Think of all the networks as files on your hard disk... You wouldn't put them all in the root directory, right? :-) By creating multiple areas, you reduce the amount of resources (memory and cpu) required to maintain the routing table.

The other is summarization. In OSPF, you can't hide subnets and only advertise a summary route to an OSPF neighbor in the same area. So if you want to summarize, you need multiple areas.


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amigan_99Network EngineerAuthor Commented:
Thanks Don.  So from a design standpoint - where do you draw the line?  I have a mix of 2811s and 3845s at the cores.
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amigan_99Network EngineerAuthor Commented:
Thanks Don.
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