This feature is used to track the state of an object for reachability using IP SLA. A client process such as Virtual Router Redundancy Protocol (VRRP), or RIB, registers itself to track objects and then get notified when a state change occurs.

IP SLA (Service-Level Assurance Protocol) is a Performance Measurement protocol. The protocol is used to Analyze IP Service Levels for IP applications and services. IP SLA's uses active traffic-monitoring technology to monitor continuous traffic on the network. 

IP SLA uses Internet Control Message Protocol (ICMP) pings to identify a link failure and notifies to the clients that are registered for tracking.

The Object Tracking feature provides complete separation between the objects to be tracked and the action to be taken by a client when a tracked object state changes. Thus, several clients such as VRRP, or RIB can register their interest with the tracking process, track the same object, and each take different action when the object changes. The Tracking feature is present in OAMD.

Each tracked object is identified by a unique number that is specified on the tracking CLI. Client processes use this number to track a specific object.

The tracking process processes events from the tracked objects and notes any change of value. The changes in the tracked object are communicated to interested client processes, either immediately or after a specified delay. The object values are reported as either up or down.

To configure VRRP Object Tracking, the object is configured to have a priority-delta value, which is passed to VRRP when a failure occurs. The priority of Virtual Router on the circuit is decremented by the priority- delta value, causing the VR Master to VR Backup transition.

In this example, two routers, R1 and R2, are configured as Master and backup routers with different priorities respectively. The priority-delta value is configured to be greater than the difference of both the priorities. R1 is configured to have a priority of 150, and R2 has a priority of 100. R1, with a greater priority, is the Virtual Router Master. The priority-delta value is 60, greater than 50 (150 minus 100). On R1, when the external interface xe41 fails, the Track state becomes DOWN and the priority of R1 becomes 90 (150 minus 60). Since R2 has a greater priority (100) than R1, R2 becomes the VR Master, and routing of packages continues without interruption. When the track state comes UP this VR Backup (R1) is UP again, it regains its original priority (150), and becomes the VR Master again.