OSPF

• OSPF (“Open Shortest Path First”) runs on each participating router (Cisco, zebra, etc).

• It announces that router's adjacencies and listens for announcements from neighbours; that allows it to build up a global “who is connected with whom” view, including multiple (redundant, potentially load-balanced, potentially unequal bandwidth) paths.

• Using that view, each OSPF router calculates the shortest path to each other OSPF router.

• OSPF is a link state protocol: recalculates after each interface transition.