OSPF

Stands for Open Shortest Path First

Uses the Shortest Path First algorithm of Dutch computer scientist Edsger Dijkstra (aka Dijkstra's algorithm).

Three versions:

• OSPFv1 (1989): deprecated, not in use anymore.

• OSPFv2 (1998): Used for IPv4.

• OSPFv3 (2008): Used for IPv6.

Routers store information about the network in LSAs (Link State Advertisements), which are organized in a structure called the LSDB (Link State Database).

Routers will flood LSAs until all routers in the OSPF area develop the same map of the network (LSDB).

LSA Flooding

OSPF is enabled on R4's G1/0 interface.

R4 creates an LSA to tell it's neighbors about the network on G1/0.

The LSA is flooded throughout the network until all routers have received it.

This results in all routers sharing the same LSDB.

Each router then uses the SPF algorithm to calculate its best route to 192.168.4.0/24.

Each LSA has an aging timer (30 min by default). The LSA will be flooded again after the timer expires.

OSPF Steps

In OSPF, there are three main steps in the process of sharing LSAs and determining the best route to each destination in the network.

1. Becoming neighbors with other routers connected to the same segment.

2. Exchange LSAs with neighbor routers.

3. Calculate the best routes to each destination, and insert them into the routing table.