Using Autocalculation

OSPF can automatically calculate the cost of an interface. This algorithm is based on the amount of bandwidth that each interface type supports. The sum of the calculated values of all interfaces in a given route forms the basis for OSPF routing decisions. These values enable OSPF to calculate routes based, at a minimum, on the bandwidth available per link in redundant routes. Figure 12-9 presents a sample network to demonstrate this point.

Figure 12-9: Autocalculated costs of the links.

In Figure 12-9, the cost of the WAN route between a host in network and an end system in network is 138. This cost is the sum of the two T1 links between those networks, each with a cost of 64, plus the cost of the Ethernet interface to network The cost of the Ethernet interfaces at the origination and destination points are not included in the OSPF cost calculation because the OSPF calculates only the costs of outbound router interfaces.

Table 12-1 summarizes the automatically calculated costs for each of the interfaces used in Figure 12-9's network diagram.

Table 12-1: Calculated Costs Per Interface Type

Interface Type

Calculated Cost

100-Mbps FDDI


10-Mbps Ethernet


1.544-Mbps T1 serial link


56-kbps serial link


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