OSPF builds a complete topological map by exchanging Link State Advertisements. This chapter walks through neighbour formation, the router ID, the DR/BDR election on multi-access segments, and a complete single-area configuration.
OSPF routers exchange Hello packets every 10 seconds (every 30 on non-broadcast media). Two routers become neighbours after exchanging Hellos with matching area, hello/dead timers, authentication, MTU, and subnet. Once full, they share complete LSDBs and run Dijkstra to compute the best path to every prefix.
Each router has a 32-bit Router ID, written as a dotted-decimal IP. Selection priority: explicit 'router-id', then highest IP on a loopback, then highest IP on any active interface. Always set Router ID explicitly with 'router-id' so renumbering interfaces does not change router identity.
3 more sections in this chapter, plus a 3-question quiz and a hands-on lab.
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