Priority of Routes

There are four levels at which OSPF considers routes.

Level 1: Paths internal to an OSPF area are always preferred.

Level 2: Routes from within the OSPF network, but from other areas (IA Routes).

Level 3: OSPF can take external routes from outside the AS. An External Type 1 link assumes that the external cost (for example in a RIP network) is similar to the internal cost.

Level 4: OSPF can take external routes from outside the AS as an External Type 2. The assumption is that this type of link’s metric should be based on its external cost. For example, with a BGP connection to the internet, the external hop count is more significant than the internal routing metric.