An OSPF restriction requires all areas to be directly connected to the backbone area so that packets can be properly routed. All packets are routed first to the backbone area by default. Packets that are destined for an area other than the backbone area are then routed to the appropriate area border router and on to the remote host within the destination area.
In large networks with many areas, in which direct connectivity between all areas and the backbone area is physically difficult or impossible, you can configure virtual links to connect noncontiguous areas. For example, Figure 55 shows a virtual link between a noncontiguous area and the backbone area through an area connected to both.
Figure 55: OSPF Topology with a Virtual Link
In the topology shown in Figure 55, a virtual link is established between area 0.0.0.3 and the backbone area through area 0.0.0.2. All outbound traffic destined for other areas is routed through area 0.0.0.2 to the backbone area and then to the appropriate area border router. All inbound traffic destined for area 0.0.0.3 is routed to the backbone area and then through area 0.0.0.2.