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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
Switch Management and Operating Concepts 129
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.