Case Study: Which Area Should This Network Be In?
Sometimes you may find that a given remote router has been, or needs to be, dual-homed to routers in different areas, as shown in Figure 5-12.
Figure 5-12. Remote Dual-Homed into Two Different Areas
When dual homing a remote site to two routers in different areas, you need to decide which area to put each link in. Begin by putting the serial link between Routers D and B in area 1 and the serial link between Routers D and C in area 2; these changes are illustrated in Figure 5-13. Now that you have those two down, you still have one small enigma to handle: Which area do you put the remote Ethernet in?
If you put the Ethernet link in area 1, Router C will route traffic to the Ethernet link completely through the core of the network to reach it—talk about suboptimal routing! Putting the Ethernet in area 1 also defeats the purpose of dual homing the remote. Since traffic can't travel from area 1 (the Ethernet) through area 2 (the serial link between Routers D and C) to area 0, the dual homing doesn't provide any redundancy.
All of these same problems apply in the opposite direction if you put the remote Ethernet in area 2.
Figure 5-13. Adding the Two Serials to Areas 1 and 2
A Third Area and Virtual Links
One possibility is to put this Ethernet in a third area (for example, area 3) and run virtual links between area 3 and area 0 through both area 1 and area 2. This will work, but it also presents a major suboptimal routing problem.
Suppose that a host attached to Router C wants to reach a destination on this remote Ethernet. Because all traffic between areas must pass through area 0, the packets would be passed to area 0 by Router C, then back to Router D, and, finally, to their destination.
Using redistribute connected to Advertise the Remote Network
One possible solution for this type of a problem is to simply redistribute the Ethernet into both areas from Router D. It's simple enough to configure:
!
hostname D
!
router ospf 10
net 10.45.8.0 0.0.0.255 area 1
net 10.45.9.0 0.0.0.255 area 2
redistribute connected
Don't Dual-Home Remotes into Different Areas
Finally, you could find some way to connect this remote site so the problem doesn't exist. This is the best solution because it doesn't add externals into the mix, and there aren't any problems with suboptimal routing. If possible, avoid remotes that are dual-homed into two different areas. Instead, find a way to connect any remote sites that need to be dual-homed to routers in the same area.