Explains IS-IS penalties for link delay anomalies, covering detection with SR-PM, automatic metric adjustments, advertisement and restoration processes, node reload behavior, and configuration guidance for managing IS-IS responses to link delay events.
A penalty for link delay anomaly is a network routing mechanism that
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automatically increases the IS-IS metric when a delay anomaly is detected
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advertises the updated metric in the network to steer traffic away from the affected link, and
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restores the original metric when the link recovers and the anomaly bit is reset.
Link anomaly detection overview
When Link Anomaly Detection is enabled in Segment Routing Performance Measurement (SR-PM), the Performance Measurement (PM) module monitors links for delay anomalies. If a delay anomaly is detected, PM sets an anomaly bit (A-bit) for the affected link.
IS-IS metric adjustment and advertisement
Upon detecting an anomaly (A-bit set), the IS-IS protocol automatically increases the link’s metric by a user-defined penalty amount. This higher metric is then advertised to the network, making the link less attractive for routing decisions and steering traffic away from the problematic link.
Recovery and metric restoration
When the link recovers and delay metrics return to normal, the PM module clears the anomaly bit. IS-IS then restores the metric to its default or previously configured value, allowing the link to be used normally in routing again.
Node reload behavior
If a network node is reloaded, the IS-IS protocol advertises the default or configured metric (without penalty) for the link until a new performance measurement is made and any anomalies are reassessed.
Configuration guidance
For information about configuring link delay anomaly threshold values and more implementation details, see the "Link Anomaly Detection with IGP Penalty" topic in the Segment Routing Configuration Guide.