Dell S4820T Configuration Manual page 654

Figure 97. 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.
Area Border Router (ABR)
Within an AS, an area border router (ABR) connects one or more areas to the backbone.
The ABR keeps a copy of the link-state database for every area it connects to, so it may keep multiple
copies of the link state database. An ABR takes information it has learned on one of its attached areas and
can summarize it before sending it out on other areas it is connected to.
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Open Shortest Path First (OSPFv2 and OSPFv3)