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Well like ordinary stub areas, it doesn't flood the area with other area routes. Instead a default route is generated toward the ABR.
Unlike an ordinary stub area, a NSSA allows you to inject external routes into it, via an ASBR.
A Cisco description of a NSSA: http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/6208-nssa.html
