IS-IS: Metrics

IS-IS defines four types of metrics:

  • Default: Required to be supported by all IS-IS implementations, usually relates to the bandwidth of the link (higher means slower)
  • Delay: Relates to the transit delay on the link
  • Expense: Relates to the monetary cost of carrying data through the link
  • Error: Relates to the residual bit error rate of the link

Each of these are evaluated independently in the SPF calculation, resulting in four SPF trees, most IS-IS implementations will only support the default metric and therefore have only a single SPF calculated tree based off this metric reducing memory and cpu overhead.

Cisco IS-IS defaults all links to a value of 10 regardless of bandwidth, this value does not change and would need to be statically changed using the isis metric interface subcommand.  Other metrics that may be configured are advertised as unsupported.

Standard or narrow metrics only supported a metric value between 1-63, wide metrics were introduced for higher bandwidth links and supports 10 bits or 1-1023 metric value.


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