HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual page 652

IP Routing—Configuring RIP, OSPF, BGP, and PBR
Configuring OSPF
13-38
Area 1
Network 1
Internal
Router C
Network 2
Figure 13-7. OSPF Network with WAN as Area 0
If these routers are the only routers at the remote sites or if the remote sites
are quite small, you could leave the network undivided. (A general rule is that
an area should include fewer than 50 routers.) In this case, all networks would
be defined as part of area 0. (See Figure 13-8.)
Site A
Network 1
Figure 13-8. OSPF Network with One Area
When you configure a router to run OSPF, you should also consider the type
of network. WAN networks are point to point, which means that your router
will synchronize its database with its one neighbor. However, your router may
also be acting as a DR, BDR, or other member in the multicast LAN
environment.
Area 0
ABR A ABR B
Area 0
Router A Router B
Area 2
Network 3
Internal
Router D
Network 4
Site B
Network 2