What is the WAN technology that you are using, if it is FR or leased lines, then your design is ok and there won't be routing loops or any kind of problems (in MPLS VPNs some problems might introduce due to the redistributions done on the provider side), but didn't you thought about having location B is area O and the other locations as separate Areas connected to Area 0 as a best OSPF practice.
Using areas has many benefits, mainly boosting the convergence time and reducing the routing table size on all the routers, its not a must in your case as you don't have that amount of routers, but it is a best practice, you might think about it.
Depending on the size of your network 20+ you can have area 0 (Backbone, and depending on how far you want to summarize and clean up extra processing power, you have ABR (DG) @the core, and links to the areas (connected to area0 or via VL's). These areas can run NSSA pointing to the stub area area0, this also reduces database table size depending on the routers you will implement in this design
I am using around 15 routers, Cisco 2600 and 1700 series. If I were to implement 2 areas, I will put all the routers under one ISP in area0 and others in area1. Do I need to configure ABR at 2 locations for backup? like one at locationD and another at locationB?
Depending on your situation you can either use a Single Border Router design or a Multiple Border Router design.
What you are talking about is the latter,which alieviates the single point of failure you eluded to,this allows BGP to add redundancy. Remember when designing make sure that the IBGP(Core) is running full mesh.
Also note that the enterprise should originate a default route for each border router which is injected into BGP to be redistributed throughout the network.
Not to sound like broken record but I like this design idea because it does give your organization redundancy.You simply troubleshoot your igp connection at each location(running OSPF).So the network receives prfix updates once recieved on the network.jRemember ibgp must run in a Full mesh remember:BGP avoide routing loops by blocking our of the interface it was learned on. Later Have agreat day
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