The router then uses
this tree to route network traf?¬?c. Figure 4-3 represents the Router A view of the network, where
Router A is the root and calculates pathways assuming this view.
Each router has its own view of the topology, even though all of the routers build a shortest-path
tree using the same link-state database.
The cost, or metric, of an interface is an indication of the overhead that is required to send packets
across a certain interface. The interface cost is inversely proportional to the bandwidth, so a higher
bandwidth indicates a lower cost. There is more overhead, higher cost, and more time delays
involved in crossing a T1 serial line than in crossing a 10-Mbps Ethernet line.
NOTE OSPF DRs and BDRs are discussed in the Cisco Certi?¬?ed Networking Professional
(CCNP) curriculum.
NOTE OSPF special areas such as stub areas are discussed in the CCNP curriculum.
144 Chapter 4: Single-Area OSPF Implementation
Figure 4-3 SPF Algorithm for Route Selection
The formula used to calculate OSPF cost is as follows:
cost = reference bandwidth / interface bandwidth (in bps)
The default reference bandwidth is 108, which is 100,000,000 or the equivalent of the bandwidth
of FastEthernet.
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