IS-IS Metrics
What the exam tests
Metric questions cover narrow versus wide metrics, default values, per-level metric configuration and the effect of metric style on path selection.
Core rules
- The narrow metric is 6 bits, range 0-63, with a default of 10 per interface.
- The wide metric is 24 bits, range 0-16,777,215, carried in extended reachability TLVs.
- ISO 10589 defines default, delay, expense and error metrics, but IP integrated IS-IS uses the default metric.
- Metrics can be set per level on an interface.
- All routers should use a compatible metric style; Nokia SR OS supports wide-metrics-only to force wide-only operation.
- Passive interfaces advertise their prefix without forming adjacencies.
Worked example
Path A is 30 + 30 = 60 and path B is 40 + 40 = 80. Narrow metrics handle both. If a link needs a cost of 70, it cannot be represented in narrow format; enabling wide metrics allows 70 and the path comparison changes. A router configured wide-metrics-only will not advertise narrow-style reachability, so mixed-style designs can produce suboptimal or missing paths.
Traps
Exceeding 63 with narrow metrics; forgetting the default cost of 10; setting a metric on only one level; mixing narrow and wide styles without planning; expecting a passive interface to form an adjacency.