Re OSPF area

Hi all,

 could someone please advice me on the maximum number of routers that can
      be in an area, and how many area can be in an AS.

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I don't beleave that this will be a problem if you arn't  hosting a $1,000,000 data centre :).
gbollydAuthor Commented:
Thanks 22chris22, some are born to be genius, and some are born to think they know all.

I will be grateful if you can answer my question if you have any idea,  rather than sarcastic.

Hi gbollyd,

From the academic side there was never a theoretical limit (not one that I every saw any way), but one would suppose at some point due to the speed of the equipment vs the number of internal LSAs and external LSUs to other AS(es) there would be a significant slowdown of general routing that the practice of utilizing a large number of routers in one AS would be prohibited.  

But on a more practical side, anyone that has such a big network would ultimately split that up into a backbone AS 0 and several adjcent areas.  This would ensure that routes on an AS would be valid and active, any route summaries out to AS 0 would be small and accurate.  This would then make for a non latent network topology which everyone strives for.

oh second part on how many areas can there be for a OSPF AS.  Again there does not seem to be an academic limit, the only stipulation is that there be a AS 0 (Backbone) and all other areas just be connected to AS 0.  But then it comes down to just how big a network do you have.  And  you are using OSPF to fit your topology to enable proper routing of your packets; not the other way around.  


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The most simplist approach is this, for every class "C" subnet you have in the network, you need at least one router.  A class "C" is the third number in the IP address -  a.b.c.d.  like 192.168.1.x  (class C is 1).

So if your IP domains have 5 different numbers in the 3rd position of the IP address, you will need at least 5 routers, likely 10.  There is NO limit to the number of routers, as long as they dont fight with each other.
Keith AlabasterEnterprise ArchitectCommented:

This is (one of) the RFC's for OSPF. It indicates that the limitation is not set by the number of devices themsleves but is more likely to be hit by the size of database that could potentially be created to host all of its records. I do not recall ever seeing a specific value or algorythm to determine the maximum number of devices within an AS for example.

actually, come to think of it, there would only be about 6-12 companies out there in the world that can give OSPF (or EIGRP) a run for its money in this scenario anyway.  But it also assumes that their infrastructure is unified (and is mostly using routable protocols) on an international scale. e.g. mainframes which can do IP (SNA??) but may not be purposely isolated.

hate it when I'm 1/2 asleep and am tyyping

I mean to say
mainframes which can do IP (SNA??) but may be purposely isolated.
Well very  interesting question..

  You will never know the exact answer..
Due to the use of the OSPF algorithm and its demand on router resources it is necessary to keep the number of routers at 50 or below per OSPF area. Areas with unreliable links will therefore require many recalculations and are best suited to operate within small areas.

 But this is not the thumb rule..
Read this article.. and judge the limitations yourself.

  The PDF file is located at...
Keith AlabasterEnterprise ArchitectCommented:
Thank you :)
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