OSPF

Routers need routes before they can forward packets beyond directly connected networks. Static routes can work in small, stable designs. They become harder to maintain when links fail, paths multiply, or many routers must agree on reachability.

Open Shortest Path First, or OSPF, distributes routing information inside one autonomous system. It is an interior gateway protocol and a link-state protocol. Instead of sending a finished routing table to every neighbor, each router describes parts of the topology with link-state advertisements.

Think of OSPF as a repeated agreement process:

discover neighbors
        ↓
form selected adjacencies
        ↓
exchange and flood LSAs
        ↓
build a link-state database
        ↓
calculate shortest paths
        ↓
install selected routes

Routers in the same area aim to hold the same link-state database. Each router then runs the shortest-path calculation with itself as the root. The result is local: two routers use the same topology information but calculate from different starting points.

The three planes to keep separate

OSPF is easier to reason about when you separate three related objects.

The neighbor table records other OSPF routers discovered on connected links. A neighbor relationship does not always become a full adjacency.

The link-state database, or LSDB, stores the LSAs known for an area. The LSDB is the topology input to the shortest-path calculation.

The routing table contains usable paths selected after OSPF calculates routes and the router compares them with routes from other sources. The routing table is not a copy of the LSDB.