Open Shortest Path First (OSPF)
The OSPF protocol is a intradomain routing algorithm suggested as an improvement on RIP for ISP. This is a link-state algorithm that uses hierarchy to do route summarization. It operates over one autonomous system.
In OSPF a single router is selected as the backbone router this is where all AS externally facing routers connect to.
Routers withing the AS broadcast Link state advertisements (LSA) which include that routers neighbours. This is circulated to the whole network and is repeated at a set interval (normally 30 minutes). Then every router using itself as the root uses Dijkstra’s algorithm to find the shortest path between itself and every other subnet.