Dell S4048–ON Configuration Manual page 579

Figure 102. 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.
An ABR can connect to many areas in an AS, and is considered a member of each area it connects to.
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Open Shortest Path First (OSPFv2 and OSPFv3)