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. Â 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.
Advertisements
