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Dell Configuration Guide for the S4048–ON System 9.14.2.6

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Track a Metric Threshold

If you configure a metric threshold to track a route, the UP/DOWN state of the tracked route is determined by the current metric for the route entered in the routing table.

To provide a common tracking interface for different clients, route metrics are scaled in the range from 0 to 255, where 0 is connected and 255 is inaccessible. The scaled metric value communicated to a client always considers a lower value to have priority over a higher value. The resulting scaled value is compared against the threshold values to determine the state of a tracked route as follows:
  • If the scaled metric for a route entry is less than or equal to the UP threshold, the state of a route is UP.
  • If the scaled metric for a route is greater than or equal to the DOWN threshold or the route is not entered in the routing table, the state of a route is DOWN.

The UP and DOWN thresholds are user-configurable for each tracked route. The default UP threshold is 254; the default DOWN threshold is 255. The notification of a change in the state of a tracked object is sent when a metric value crosses a configured threshold.

The tracking process uses a protocol-specific resolution value to convert the actual metric in the routing table to a scaled metric in the range from 0 to 255. The resolution value is user-configurable and calculates the scaled metric by dividing a route's cost by the resolution value set for the route type:
  • For intermediate system to intermediate system (ISIS), you can set the resolution in the range from 1 to 1000, where the default is 10.
  • For OSPF, you can set the resolution in the range from 1 to 1592, where the default is 1.
  • The resolution value used to map static routes is not configurable. By default, Dell EMC Networking OS assigns a metric of 0 to static routes.
  • The resolution value used to map router information protocol (RIP) routes is not configurable. The RIP hop-count is automatically multiplied by 16 to scale it; a RIP metric of 16 (unreachable) scales to 256, which considers the route to be DOWN. For example, to configure object tracking for a RIP route to be considered UP only if the RIP hop count is less than or equal to 4, you would configure the UP threshold to be 64 (4 x 16) and the DOWN threshold to be 65.

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