Concept Guide

A tracked route matches a route in the routing table only if the exact address and prefix length match an entry in the routing
table. For example, when configured as a tracked route, 10.0.0.0/24 does not match the routing table entry 10.0.0.0/8. If no
route-table entry has the exact address and prefix length, the tracked route is considered to be DOWN.
In addition to the entry of a route in the routing table, you can configure how the status of a route is tracked in either the
following ways:
● By the reachability of the route's next-hop router.
● By comparing the UP or DOWN threshold for a route’s metric with current entries in the route table.
Track Route Reachability
If you configure the reachability of an IP route entry as a tracked object, the UP/DOWN state of the route is determined by the
entry of the next-hop address in the ARP cache.
A tracked route is considered to be reachable if there is an address resolution protocol (ARP) cache entry for the route's
next-hop address. If the next-hop address in the ARP cache ages out for a route tracked for its reachability, an attempt is made
to regenerate the ARP cache entry to see if the next-hop address appears before considering the route DOWN.
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.
Set Tracking Delays
You can configure an optional UP and/or DOWN timer for each tracked object to set the time delay before a change in the state
of a tracked object is communicated to clients. The configured time delay starts when the state changes from UP to DOWN or
the opposite way.
If the state of an object changes back to its former UP/DOWN state before the timer expires, the timer is cancelled and the
client is not notified. If the timer expires and an object’s state has changed, a notification is sent to the client. For example, if
the DOWN timer is running when an interface goes down and comes back up, the DOWN timer is cancelled and the client is not
notified of the event.
If you do not configure a delay, a notification is sent when a change in the state of a tracked object is detected. The time delay
in communicating a state change is specified in seconds.
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Object Tracking