OSPF Design - When to create new instance

When designing an OSPF topology, it is important to understand when to create a single instance with a single area, a single instance with multiple areas, and when to create multiple instances of OSPF domains.

In a topology where remote sites are interconnected with some WAN technology that delivers its own routing infrastructure, such as a BGP network, then each remote site should have its own OSPF instance. In other words, each remote site should have an independent area 0 backbone area. This is the case because each remote site with an OSPF instance will redistribute its routes into the WAN routing domain, and the WAN domain will redistribute them back into the other remote sites, thus each remote location will learn about the routes to the other remote sites via the WAN routing infrastructure.

There is however one exception to this arrangement, and that is when an MPLS network is used as the WAN network, and OSPF is used between the CE and PE routers. In such a case, OSPF will view the MPLS network as an MPLS SuperBackbone.




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