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Understanding Wide IS-IS Metrics for Traffic Engineering

All OSPF and IS-IS interfaces have a cost, which is a routing metric that is used in the link-state calculation. Routes with lower total path metrics are preferred over those with higher path metrics. Unlike OSPF, in which the link metric is calculated automatically based on bandwidth, there is no automatic calculation for IS-IS. All IS-IS links use a metric of 10 by default.

Normally, IS-IS metrics can have values up to 63. The total cost to a destination is the sum of the metrics on all outgoing interfaces along a particular path from the source to the destination. By default, the total path metric is limited to 1023. This metric value is insufficient for large networks and provides too little granularity for traffic engineering, especially with high-bandwidth links. A wider range of metrics is also required if route leaking is used.

IS-IS generates two type, length, and value (TLV) tuples, one for an IS-IS adjacency and the second for an IP prefix. To allow IS-IS to support traffic engineering, a second pair of TLVs has been added to IS-IS, one for IP prefixes and the second for IS-IS adjacency and traffic engineering information. With these TLVs, IS-IS metrics can have values up to 16,777,215 (224 – 1).

By default, Junos OS supports the sending and receiving of wide metrics. Junos OS allows a maximum metric value of 63 and generates both pairs of TLVs. To configure IS-IS to generate only the new pair of TLVs and thus to allow the wider range of metric values, you must include the wide-metrics-only statement in the IS-IS configuration.The combination of wide-metrics-only and traffic-engineering disable configuration options under IS-IS protocols suppresses the combination of the TLVs 2, 22, 128, 134, and 135 IS-IS routing information for that level. That means that the local server will not send the TLVs but accepts them when received. The effect of the configuration options on TLVs 2, 22, 128, 134, and 135 will be individually evaluated.

See [Understanding the effects of ISIS wide-metric-only and traffic-engineering disable configuration options in Junos.]