The Backbone and Area 0
OSPF has special restrictions when multiple areas are
involved. If more than one area is configured, one of these areas has to be
area 0. This is called the backbone.
The backbone has to be at the center of all other areas, every
other areas have to be physically connected to the area 0 (backbone area). The
reasoning for this is that OSPF expects all areas to inject routing information
into the backbone and in turn the backbone will spread out that information
into other areas. When designing networks it is good practice to start with
area 0 and then expand into other areas later on.
If it is not possible to connect area to the backbone area 0 there is solution for that, we have tool that can be used, and that would be virtual link.
Virtual links are used for two purposes:
- Linking an area that does not have a physical connection to the backbone.
- Patching the backbone in case discontinuity of area 0 occurs.
