OSPF Area Border Connection Behavior
Reference:
Designing Cisco Network Service Architectures ARCH Foundation Learning Guide - Cisco Press
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.
Summary:
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
