(also ABR)
In the Open Shortest Path First (OSPF) protocol, area border routers connect OSPF areas to the backbone network (also known as “area 0” or “0.0.0.0”). Area border routers serve to provide summaries of the sub networks throughout a given OSPF system.
As the name suggests, area border routers are placed on the borders of OSPF areas to serve as entry and exit points for information. Because an area border router belongs to both the areas it connects to and the backbone network, it maintains the necessary routing information for each area.