In an OSPF network, each router maintains a link-state database that describes the topology of the AS. The database contains the local state for each router in the AS, including its usable interfaces and reachable neighbors. Each router periodically checks for changes in its local state and shares detected changes by flooding link-state advertisements (LSA) throughout the AS. Routers synchronize their topological databases based on the sharing of information from LSAs.
From the topological database, each router constructs a shortest-path tree, with itself as the root. The shortest-path tree provides the optimal route to each destination in the AS. Routing information from outside the system displays the AS on the tree as leaves.
OSPF routes IP traffic based on the destination IP address, subnet mask, and IP TOS.
In large networks, OSPF offers the following benefits:
fast convergence
After network topology changes, OSPF recalculates routes quickly.
minimal routing protocol traffic
Unlike distance vector routing protocols, such as Routing Information Protocol (RIP), OSPF generates a minimum of routing protocol traffic.
load sharing
OSPF provides support for Equal Cost Multipath (ECMP) routing. If several equal-cost routes to a destination exist, ECMP distributes traffic equally among them.
type of service
OSPF can calculate separate routes for each IP TOS.