20.OSPF is a link-state routing protocol。It is designed to be runinternal to a single ( )system。Each OSPF router maintains an identical ( )describing the autonomous system’s topology。From this database,a routing table is calculated by constructing a shortest-path ( )。OSPF recalculates routes quickly in the face of topological changes,utilizing a minimum of routing ( ) traffic。OSPF provides support for equal-cost multipath。An area routing capability is provided,enabling an additional level of routing protection and a reduction in routing protocol( )。