OSPF Area Border Connection Behavior
Designing Cisco Network Service Architectures ARCH Foundation Learning Guide – Cisco Press
In the case of OSPF, area definition is on a per layer 3 interface basis.
For example, referencing the topology above, the link between router D and router F is placed in area 1
Traffic crossing the backbone must get into an area by the shortest path and then stay in that area.
In this example, the link from D to E is in area 0. If the D-to-F link fails, traffic from D to F goes from D to G to E to F. Because D is an ABR for area 1, the traffic to F is all internal to area 1 and must remain in area 1. OSPF does not support traffic going from D to E and then to F because the D-to-E link is in area 0, not in area 1.A similar scenario applies for traffic from A to F: It must get into area 1 by the shortest path through D and then stay in area 1.
In OSPF, traffic from area 1 to area 1 must stay in area 1 unless area 1 is partitioned, in which case the backbone area 0 can be used. Traffic from area 1 to area 2 must go from
area 1 to area 0, and then into area 2. It cannot go into and out of any of the areas in other sequences.
You can connect the ABRs within each area by either of two means:
- Adding a real link between the ABRs inside area 1
- Adding a virtual link between the ABRs inside area 0
With multiple logical links, whether physical, subinterfaces, or VLANs between a pair of ABRs, the following options are recommended:
- Consider making sure that a link exists between the ABRs within each area on those ABRs.
- Implement one physical or logical link per area.
In this case, you will ideally deploy two layer 3 interfaces between routers D and E.
On both routers, the first layer 3 interface will be ideally in backbone area 0.
The second layer 3 interface on both routers should then be in area 1 as shown in figure below.