01Plain English
The simple explanation
An area border router tells an area how to reach a network outside that area.
02Under the hood
The technical explanation
An ABR originates Type 3 summary-LSAs to advertise inter-area network reachability subject to OSPF’s area rules and configured policy. They are originated into an area, not flooded unchanged across every area. “Summary” does not mean routes must be aggregated.
Protocol reference: RFC 2328 (OSPFv2). NSSA reference: RFC 3101.
logic
Make it stick
The friend who turns a forty-message argument into three useful sentences.
03On the wire
Example
An ABR can advertise a network learned in Area 10 into Area 0 as a Type 3 LSA.