OSPF routes are summarized on an ABR. External routes are summarized on an ASBR.
If physical interfaces are only used, the OSPF router ID is the highest IP address assigned to an active physical interface. If loopback interfaces are used, the OSPF router ID is the highest IP address assigned to a loopback interface. If the router-id command is used with the OSPF configuration, the address used with this command will be the router ID.
The router with the highest interface priority will be the router ID. If all the interface priorities on the multi-access network are the same, the router with the highest router ID will be the DR.
First, calculate the shortest path to an ABR.
Second, calculate the shortest path across area 0 to an ABR that is attached to the destination area.
Third, calculate the shortest path across the destination area from the ABR to the destination network.
To connect a nonzero area to the backbone if the nonzero area becomes disconnected from the backbone. A virtual link can also be used if the backbone, or area 0, becomes discontiguous.