Control Plane Fundamentals
OSPF uses IP protocol number 89 to communicate between neighbours. The multicast address of 220.127.116.11 (AllSPFRouters) is used to send all control plane packets between neighbour interfaces.
OSPF speaking devices flood packets to each other using a reliable algorithm,. All devices synchronise their databases, and using the SPF algorithm each device calculates a shortest path tree to all other devices, using itself as the root of the shortest-path tree.
OSPF Router ID