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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
128 Switch Management and Operating Concepts
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