OSPF design for branch offices across MPLS

Question

Hello fellow networking engineers,
I want to implement OSPF in our network. We have multiple branch offices, all linked to an MPLS backbone.
I know that in order to get linked areas, I would need to setup GRE tunnels between them, but I want to avoid static/manual configurations as much as possible. With multiple sites, it would become cumbersome to create a mesh real fast.
Is running OSPF independent areas at each site, and simply redistributing over eBGP a valid solution? This will host voice and data, and will failover to VPN connection (Cisco ASAs) if the MPLS goes down.
For the VPN backup links, I thought of two options. Either simply using the default route to send everything to the ASA in case of MPLS "death", or inject routes using IP SLA...
Any input would be appreciated.

Answer 1

Marc
You don't GRE tunnels to link your areas if that is what you want to do.
If the SP supports it then you can exchange your OSPF routes between areas and they will still be seen as inter area routes rather than OSPF externals which they would if you simply treated each area as isolated from each other.
In effect the MPLS network becomes an OSPF super backbone area and your main site would also be part of the backbone area with all your other sites having an area each.
You still redistribute your OSPF routes into BGP but with some extra configuration on both your CEs and the SP PE devices.
Like I say you would need to check with your SP but it is possible.
Whether or not you need or want it I don't know.
Your other option is as you have proposed to treat each OSPF area as an isolated one and simply redistribute into OSPF at each CE. Then within each site all non local routes would be seen as OSPF external routes.
Either way in terms of backup I would keep it simple and use a default route at each site pointing to the ASA device. I can't see what you gain from IP SLA because if the main MPLS link goes down at any site the only other path they have out is via the ASA so there is nothing really worth tracking.
The only other thing I would mention is remote site to remote site traffic. If there is any then presumably with your VPN tunnels you would be doing a sort of hub and spoke where the hub is the main site so you may need to think about traffic coming in from one VPN tunnel and going out to another VPN tunnel on the main site ASA.
This would only really be needed if two or more sites had to use their backup links at the same time.
In terms of which is better ie. OSPF inter area across the MPLS cloud or OSPF externals I can't really say to be honest. With the MPLS networks i have worked on we ran EIGRP and simply treated each remote site as an isolated AS.
If you are already running OSPF then you may want to preserve your existing areas so it would make sense to go with the inter area option.
If it is a new setup then I don't really know the pros and cons of either so can't really comment.
Perhaps others may add to the thread with their thoughts.
Jon