Can someone explain in practical terms what is the disadvantage of running OSPF in a single area vs. multi area. I would like to have as many details as possible - I am ccnp qualified so if you feel like being technical, even better.
Unfortunately, my ospf experience in large deployment scenarios is lacking which is why I am not able to answer this question myself; it's those situations when just reading the book, it's simply not enough.
Just to emphasize, i'd like a practical answer; from the network management point of view, to the more technical point of view; tips and tricks, recommendations, caveats, etc.
Much appreciated.
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The main advantages of multiple area OSPF is route summarisation between OSPF areas if each area is subnetted properly you can reduce the size of the routing table in each area. You can also use totally stub areas where only a default route is injected into an area.
The other advantage is a topology change in one area doesn't cause an SPF calculation to happen in the other areas.
Do you have any specific questions?
Not really specific questions; the idea was rather to debate this topic. Obviously, I will share the points accordingly.
More often than not, single areas appear to be more frequent than multi areas? Why is that so? Management purposes? If multi areas are so nice as cisco teaches, why would people avoid them?
If they have a small number or routers or routes in their networks then it's probably not needed. Route summarisation seems to be the driving force for doing multi area or maybe if you've got a specific reason for doing it. You could have a slow WAN link or a stub network that doesn't need a full routing table so you just give it a default route. Some older routers, may not have enough memory to hold the ospf database and a large routing table so a totally stub area would be ideal.
What do YOU define as small and large? It's an unanswerable question. There are numerous variables at work. Number of networks, number of routers, how much memory the routers have, etc.
Kind of like the old "how big does the network have to be before you have to switch from RIP to OSPF" question.
Too many variables.
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by: donjohnstonPosted on 2009-10-06 at 15:03:08ID: 25510398
Summarization is one reason.
If you want to summarize routes in OSPF, you have to do it on an ABR. Which means multiple areas.
Another is if the area gets too big, the link state database can become large and unwieldy. Creating multiple areas effectively breaks up the database into smaller chunks.