To area or not to area in OSPF

[jkeeffe]

We have an EIGRP network with about 300 layer three devices (3750, 6500, 3845, 7604) and are starting a migration to OSPF project.

One of the big questions is whether we want to have more than Area 0.  A big reason to carve up the network into areas would be to have summarization, but that's not going to happen in our network because we've got a class-B /25 subnets scattered all over the place and will not be readressing due to extreme labor in all aspects of the idea.

So since summarization is not a reason to have multiple areas, I'm of a mind to just have one area for our OSPF environment. Other network engineers are warning against a single area, some saying there should only be 50 L3 devices in an area. My idea is to keep is simple, and I don't think with the CPU power of these devices, that having all our routes in one area is a problem.

I found the attached document quite interesting concerning areas.

Thoughts?

[fisherman_188]

I am not an OSPFe xpert but you mention "...I don't think with the CPU power of these devices, that having all our routes in one area is a problem."  The massive routing table isn't your main problem with the CPU usage, your main problem will be all the LSU's and LSAck flying around, with 300 devices, every time a change in a link is made, you network will be severely crippled or could even crash some routers.  With an area of that size there very high chance that link changes will happen often, which will result in ever single router having to run the SPF (Dijisktra) algorithm every time a change occurs, which is a CPU intensive process.  Having a single area will keep you routers overloaded with 300 devices, you you're best bet is to spend the extra time changing your addressing scheme to a more hierarchical structure now rather than deal with more problems later.   The limit of 50 routers per area is Cisco's recomendation, though there are some exceptions, 300 routers are way too many. My 2 cents.

[Faraz Shamim]

It's a crazy idea but doable take a look at the following feature of OSPF:

http://www.cisco.com/en/US/docs/ios/12_4t/12_4t15/ht_osmch.html

It also depends on what type of topology are we talking about. If its hub and spoke then 300 neighbors would be too many. If the router lsa size exceeds the interface mtu than it will use ip fragmentation which can impact the performance. If the neighbors are scattered i.e. not hub and spoke then you can push it to 300. The only issue is that prefix suppression feature may not be available in all code. Otherwise you could literally bring down the size to 500-1000 routes assuming there will be quiet a few broadcast network where your servers reside. Good luck with the design.

BTW, we have removed all the papers that used to give total number of routers you can have in an area. If you still find any paper that says that then let me know and I will get it removed.

Faraz

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