OSPF area 0 with Cisco routers

My WAN is MPLS and I am wondering how I configure OSPF with one area across my 3 sites (see pic below). I have no management of the ISP CE routers and they run EBGP to my layer 3 switches. Thx

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There can be only 1 area 0 and all other areas must be attached to area 0, so you must have that central area as area 0, but why would you do that???
If all you have only area 0, there is no summarization point anywhere. You can only summarize routes on ABRs and ASBRs.
So, you could set only that inside area as area 0 (or one of areas should be area zero and each router on edge of other areas should have one interface in area 0) and all surrounding areas could be area X and then you can do summarization on each border router for that area, you should assign IP address ranges to all areas so those could be easily summarized (also there should be no overlapping). But remember that all inter area traffic will pass through area 0 (it's a loop prevention mechanism).
Since you have Cisco routers, if you can mark some areas as stub areas (totally stuby areas - Cisco proprietary - advertisement type LSA3, LSA4, LSA5 are replaced with just default route), or no so stubby areas and totally nssa... and significantly reduce ospf traffic in those areas and OSPF time to reconverge if there are some changes outside specific area.

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leblancAccountingAuthor Commented:
I want to keep things simple for my small network. That is why I was thinking of just one area.
I understand your point about making the WAN area 0. However, the ISP only runs BGP and I have no control over the WAN routers.
So it looks like each location will have its own OSPF process. I read some white papers on OSPF and they assume that we own the WAN routers. But if I want to run OSPF for my whole network, how will it work? Is it possible? Thx
Don JohnstonInstructorCommented:
Since your provider is running BGP, unless you've got a VPN between sites, OSPF is probably not something you can do.
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Creating OSPF between all areas is challenge in your topology, but sure it can be done. It can always be done one one way or the other.
You can create GRE tunnels between areas, as Don already said, as long as your network don't grow too big it should be OK.
The biggest issue here, as I see it, is optimization of IP addresses to different physical locations.
Let's say, to keep it simple, that you assign area A - 192.168.x.x, area B -  172.16.x.x - 172.31.x.x, area C 10.x.x.x. In that case you can create just one summarization for each area (or one static route to point to area). But even summarization should be as specific as it can be, so traffic for networks that don't exist in distant areas is dropped in originated area  to save bandwidth. Sure if all areas are area 0 you don't have that problem since all networks will be known by each router, but you have other problems like one flapping interface on some router could do a lot of harm if all your locations are area 0.
GRE tunnels will make each MPLS link look like a single hop.  You can do it with or without encryption.

It does add a little overhead to the packet, so you need to use the tcp adjust-mss command to reduce the maximum segment size.

Then you can exchange routing info across the GRE interfaces any way you want.  OSPF, EIGRP, etc.
leblancAccountingAuthor Commented:
Got it. So from what I see, you have to own all the devices in your network in order to deploy OSPF in your organization including the WAN, We can indeed use GRE but that adds extra header in the packets. Does any of you deal with ISPs who deploy OSPF in their MPLS network? If yes, then I assume that they use area 0 so that their OSPF customers can connect to. Thx
Don JohnstonInstructorCommented:
So from what I see, you have to own all the devices in your network in order to deploy OSPF in your organization including the WAN,
That's pretty much it.  Unless you tunnel across the devices you don't control.
Does any of you deal with ISPs who deploy OSPF in their MPLS network?
I've never run across an MPLS provider that does any routing protocol other than BGP.  That's not to say it doesn't happen though.
There is one tiny thing ....
Since tunnel will be seen as directly connected interface and OSPF will start learn routes through tunnel, it is most likely that router will try to establish tunnel through tunnel and tunnel will colapse.
So, there is a big chance that you will need to add static routes to be able to establish working GRE tunnels, beside of making MTU and MSS smaller.
Exclude the MPLS interfaces from OSPF, and the problem goes away.

Similarly, you'd probably want to exclude internal interfaces from the MPLS/BGP routing protocol.

Alternatively, adjust the admin distance on the routing protocols.
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