While some routing protocols calculate the link metric automatically based on bandwidth (OSPF) or bandwidth/delay (Enhanced Interior Gateway Routing Protocol [EIGRP]), there is no automatic calculation for IS-IS. Using old-style metrics, an interface cost is between 1 and 63 (6 bit metric value). All links use the metric of 10 by default. The total cost to a destination is the sum of the costs on all outgoing interfaces along a particular path from the source to the destination, and the least-cost paths are preferred.

The total path metric was limited to 1023 (the sum of all link metrics along a path between the calculating router and any other node or prefix). This small metric value proved insufficient for large networks and provided too little granularity for new features such as Traffic Engineering and other applications, especially with high bandwidth links. Wide metrics are also required if route-leaking is used.

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