OSPF Area

Answered Question
Jun 5th, 2009

How many routers and users can be in a single OSPF area?

Correct Answer by Joseph W. Doherty about 7 years 8 months ago

Well one rule-of-thumb used to be about 50, but as Franco notes, with modern routers you might easily double that, but again, there are many variables involved. The less you have probably makes you "safer". If you want to be super cautious, you might want to stay under 30, yet again, your network might be fine with many, many more. Part of the consideration might be how your topology shakes out, i.e. whether there's natural hiearchy you can map your OSPF areas and/or what advantages routers having knowing the topology of their area. Also keep in mind, OSPF area borders is also where you manage network prefix summarization and/or network prefix distribution.

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Overall Rating: 4.3 (4 ratings)
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Harold Ritter Fri, 06/05/2009 - 04:12

George,


The short answer is it depends. I have seen customers running hundreds of routers in a single area. The router in this area were not low end boxes though.


Regards

francisco_1 Fri, 06/05/2009 - 04:48

I believe Cisco recommends

that you not have an OSPF area with more than 90-100 routers (Might have more routers nowadays) . Additionally

it is Cisco's recommendation that you not have more than 200 subnetworks per

an area. Again .. these are recommendations and the network topology can

have a dramatic effect on what are stable numbers for other networks.

Joseph W. Doherty Fri, 06/05/2009 - 08:31

George, I see you've marked Stephen's post with a resolution flag, which is fine, but as Harold noted in his post, "it depends". There are even more considerations than what the Cisco reference document notes under "Number of Routers per Area". Some are a bit exotic, such as the IOS you're using and what internal OSPF enhancements it supports. Some depend on IOS configuration. Much depends on topology, link stability, etc.


OSPF geneally works well until some point that pushes it "over the edge". How many routers to have in an OSPF area is somewhat akin to how many hosts you can have within a subnet.


Recommendations for X number of routers per OSPF area, with typical OSPF best practice configurations, typically work just as about as well as X number of hosts per subnet. Both often are "safe" for typical situations, but both also might not be suitable for your network.


So, be careful using any one set of recommendations/considerations.

everestmountain Fri, 06/05/2009 - 09:34

What is the maximum number of routers in an OSPF area which would be safe for most configurations?


Correct Answer
Joseph W. Doherty Fri, 06/05/2009 - 16:42

Well one rule-of-thumb used to be about 50, but as Franco notes, with modern routers you might easily double that, but again, there are many variables involved. The less you have probably makes you "safer". If you want to be super cautious, you might want to stay under 30, yet again, your network might be fine with many, many more. Part of the consideration might be how your topology shakes out, i.e. whether there's natural hiearchy you can map your OSPF areas and/or what advantages routers having knowing the topology of their area. Also keep in mind, OSPF area borders is also where you manage network prefix summarization and/or network prefix distribution.

Joseph W. Doherty Tue, 06/09/2009 - 04:45

It might if your intent is to control volume of LSAs being passed across the WAN link using an ABR. Or, you want to protect other routers from the impact of link flapping, again filtering at ABRs.

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