Dell S4820T Configuration Manual page 687

Figure 96. OSPF Routing Examples
Backbone Router (BR)
A backbone router (BR) is part of the OSPF Backbone, Area 0.
This includes all ABRs. It can also include any routers that connect only to the backbone and another
ABR, but are only part of Area 0, such as Router I in the previous example.
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Open Shortest Path First (OSPFv2 and OSPFv3)