ROUTING DECISIONS

Originally, OSPF based its routing decisions on two fields in the IP datagram: the destination IP address and the type of service (TOS). The type of service field contains several values. The values in the field that can be used by OSPF for routing are the delay, throughput, reliability, and cost fields. While the Internet Protocol and OSPF provide rules on how these fields are coded, as well as general definitions on their meaning, neither protocol dictates how the values are translated into network operations.

Thus, a network administrator was free to choose the method by which these values are employed. As OSPF found its way into commercial networks, the TOS value was not used much, and due to this very modest employment, it has fallen into disuse. Therefore, other means are now implemented to assign metrics to links and networks. We shall explore them shortly.