OSPF area design questions

Unanswered Question
Jul 29th, 2008

Hi there,

My network currently exists as a single OSPF area, spread between 3 datacenters across the country. This has worked well for us because we don't have very complex routing issues and we've had the same three sites for years. We are now in a position though, where we are thinking about opening datacenters in other locations, including foreign datacenters.

My question is about single area vs. multi-area OSPF design. I don't have a lot of routers... and will probably never have any huge number. The links between the sites are all DS3's or higher, and in some cases redundant links are installed.

Would it make sense to move to a multi area OSPF design, with each site being it's own area, or would that be overkill? I've attached two diagrams indicating what I have setup now, versus what I imagine the multi-area design to be.

I see the following as potential benefits to a multi-area setup:

-route summarization between areas simplifies routing tables

-Less routes means less LSA's between geographically seperated sites

-Loss of Area 0 WAN link doesn't cause routing reconvergence problems at each site.

Drawbacks would include:

--More complex

-- ?

Any thoughts would be appreciated.


I have this problem too.
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Giuseppe Larosa Tue, 07/29/2008 - 12:01

Hello Brandon,

provide a double link of each site area to area 0.

You can have less SPF calculations because they are bonded to each area and this is another benefit.

The question is have you got internal routers on each site that can benefit from the multiple area approach ?

If you have only the front end routers at each site migration is very simple just modify the network commands so that IP subnets internal to the site can be associated to non-backbone site-specific area.

Otherwise you need to use double links on every internal router and migrate one by one to avoid to lost control of internal routers

when changing the area number ospf adj must be rebuilt.

area number and type must match to build the ospf adj

I would do it because it will provide more scalability even if there aren't internal routers in site at the moment.

Hope to help


branfarm1 Wed, 07/30/2008 - 03:59

Thanks for the response Giuseppe.

Can you please clarify what you mean when you say "modify the network commands so that IP subnets internal to the site can be associated to non-backbone site-specific area"?



Giuseppe Larosa Wed, 07/30/2008 - 09:24

Hello Brandon,

let's use an example:

let's say that site A is going to be migrated to ospf area 100.

now you have:

router ospf 200

network area 0

prefixes within are site A specific.

you need to modify:

network area 100

network statements are like ACL statements so you may need to remove the previous command:

no network area 0

add specific entries for links to backbone and loopback address

network area 0

network area 0

where the second one is the loopback

all this under router ospf 200 config mode

Hope to help


m-haddad Wed, 07/30/2008 - 15:32

Hello Brandon,

It makes more sense to create multiple areas. This will allow you to summaries network segements on the ABRs => Less routing reconvergence in case some routes disappear from the edge routes/MLS Switches.

In order to achieve this, you need to terminate the WAN side of the routers to area 0 ,which will be the ABR, and the other side of the router with the MLS switch to area 1 for example. Then you can do route summarizations and LSA filtering between areas on the ABRs.

Hope this helps,

Appreciate your rating,


Danilo Dy Fri, 08/01/2008 - 09:49

multi-area makes more sense, more control locally.

I also suggest using ospf locally and bgp accross country/other locations.

branfarm1 Fri, 08/01/2008 - 10:15

hi- thanks for your reply.

would you still recommend Bgp if the wan links were virtual layer 2 links? essentially, all of our sites- both in the US and foreign- will be on the same LAN (in the WAN).

Also, since I don't have a working knowledge of BGP yet, could you please explain the potential benefits of using BGP between our sites?

Thanks in advance,


branfarm1 Mon, 08/04/2008 - 21:31

also, is there any way to load balance traffic over the existing DS3's and the new layer two links, if I change the ospf design?

Thanks in advance,



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