Object Tracking | 679Tracking Layer 3 InterfacesYou can create an object that tracks the Layer 3 state (IPv4 or IPv6 routing status) of an interface.• The Layer 3 status of an interface is UP only if the Layer 2 status of the interface is UP and theinterface has a valid IP address.• The Layer 3 status of an interface goes DOWN when its Layer 2 status goes down or the IP address isremoved from the routing table.Tracking IPv4 and IPv6 RoutesYou can create an object that tracks an IPv4 or IPv6 route entry in the routing table. You specify a trackedroute by its IPv4/IPv6 address and prefix-length, and optionally, by a VRF instance name if the route to betracked is part of a VRF. The next-hop address is not part of the definition of the tracked object.A tracked route matches a route in the routing table only if the exact address and prefix length match anentry in the routing table. For example, when configured as a tracked route, 10.0.0.0/24 does not match therouting table entry 10.0.0.0/8. If no route-table entry has the exact address and prefix length, the trackedroute 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 trackedin 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 tableTracking Route ReachabilityIf you configure the reachability of an IP route entry as a tracked object, the UP/DOWN state of the routeis determined by the entry of the next-hop address in the ARP cache. A tracked route is considered to bereachable if there is an ARP cache entry for the route's next-hop address. If the next-hop address in theARP cache ages out for a route tracked for its reachability, an attempt is made to regenerate the ARP cacheentry to see if the next-hop address appears before considering the route DOWN.Tracking a Metric ThresholdIf you configure a metric threshold to track a route, the UP/DOWN state of the tracked route is determinedby 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 0 to 255,where 0 is connected and 255 is inaccessible. The scaled metric value communicated to a client alwaysconsiders a lower value to have priority over a higher value. The resulting scaled value is compared againstthe 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.
