D-Link DES-3326S manual Shortest Path Algorithm, 128

DES-3326S Layer 3 Fast Ethernet Switch User’s Guide

1.When OSPF is started, or when a change in the routing information changes, the router generates a link-state advertisement. This advertisement is a specially formatted packet that contains information about all the link- states on the router.

2.This link-state advertisement is flooded to all router in the area. Each router that receives the link-state advertisement will store the advertisement and then forward a copy to other routers.

3.When the link-state database of each router is updated, the individual routers will calculate a Shortest Path Tree to all destinations − with the individual router as the root. The IP routing table will then be made up of the destination address, associated cost, and the address of the next hop to reach each destination.

4.Once the link-state databases are updated, Shortest Path Trees calculated, and the IP routing tables written − if there are no subsequent changes in the OSPF network (such as a network link going down) there is very little OSPF traffic.

The Shortest Path Algorithm

The Shortest Path to a destination is calculated using the Dijkstra algorithm. Each router is places at the root of a tree and then calculates the shortest path to each destination based on the cumulative cost to reach that destination over multiple possible routes. Each router will then have its own Shortest Path Tree (from the perspective of its location in the network

128 Switch Management and Operating Concepts