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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
The Link-State Algorithm
An OSPF router uses a link-state algorithm to build a shortest path tree to all destinations known to
the router. The following is a simplified description of the algorithm’s steps:
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 area) even though every router in
the area will have and use the exact same link-state database.
The following sections describe the information used to build the Shortest Path Tree.
OSPF Cost
Each OSPF interface has an associated cost (also called “metric”) that is representative of the overhead
required to send packets over that interface. This cost is inversely proportional to the bandwidth of the
interface (i.e. a higher bandwidth interface has a lower cost). There is then a higher cost (and longer
time delays) in sending packets over a 56 Kbps dial-up connection than over a 10 Mbps Ethernet
connection. The formula used to calculate the OSPF cost is as follows:
Cost = 100,000,000 / bandwidth in bps
As an example, the cost of a 10 Mbps Ethernet line will be 10 and the cost to cross a 1.544 Mbps T1
line will be 64.
Shortest Path Tree
To build Router A’s shortest path tree for the network diagramed below, Router A is put at the root of
the tree and the smallest cost link to each destination network is calculated.
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