Reference Guide

Open Shortest Path First (OSPFv2 and OSPFv3) | 659
Figure 35-1. Autonomous System Areas
Area Types
The Backbone of the network is Area 0. It is also called Area 0.0.0.0 and is the core of any Autonomous
System (AS). All other areas must connect to Area 0. Areas can be defined in such a way that the backbone
is not contiguous. In this case, backbone connectivity must be restored through virtual links. Virtual links
are configured between any backbone routers that share a link to a non-backbone area and function as if
they were direct links.
An OSPF backbone is responsible for distributing routing information between areas. It consists of all
Area Border Routers, networks not wholly contained in any area, and their attached routers.
The Backbone is the only area with an default area number. All other areas can have their Area ID assigned
in the configuration.
Figure 35-1 shows Routers A, B, C, G, H, and I are the Backbone.
Router A
Router D
Router B
Router C
Router E
Router F
Router
G
Router H
Router I
Router J
Router L
Router K
Router M
Area 100
Area 200
Area 300
Area 0