First Hop Redundancy Protocols Configuration Guide, Cisco IOS XE Release 3S
VRRP Object Tracking
Object tracking is an independent process that manages creating, monitoring, and removing tracked objects such as the
state of the line protocol of an interface. Clients such as the Hot
Standby Router Protocol (HSRP), Gateway Load Balancing Protocol
(GLBP), and VRRP register their interest with specific tracked objects
and act when the state of an object changes.
Each tracked object is identified by a unique number that is specified
on the tracking CLI. Client processes such as VRRP use this number to
track a specific object.
The tracking process periodically polls 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
VRRP object tracking gives VRRP access to all the objects available
through the tracking process. The tracking process allows you to track
individual objects such as a the state of an interface line protocol,
state of an IP route, or the reachability of a route.
VRRP provides an interface to the tracking process. Each VRRP group
can track multiple objects that may affect the priority of the VRRP
device. You specify the object number to be tracked and VRRP is
notified of any change to the object. VRRP increments (or
decrements) the priority of the virtual device based on the state of
the object being tracked.
How VRRP Object Tracking Affects the Priority of a Device
The priority of a device can change dynamically if it has been
configured for object tracking and the object that is being tracked
goes down. The tracking process periodically polls the tracked objects
and notes any change of value. The changes in the tracked object are
communicated to VRRP, either immediately or after a specified delay.
The object values are reported as either up or down. Examples of
objects that can be tracked are the line protocol state of an
interface or the reachability of an IP route. If the specified object
goes down, the VRRP priority is reduced. The VRRP device with the
higher priority can now become the virtual device master if it has the
vrrp preempt command configured. See the “VRRP Object Tracking”
section for more information on object tracking.
The virtual router master sends VRRP advertisements to
other VRRP routers in the same group. The
advertisements communicate the priority and state of the virtual
router master. The VRRP advertisements are encapsulated in IP packets
and sent to the IP Version 4 multicast address assigned to the VRRP
group. The advertisements are sent every second by default; the
interval is configurable.
Although the VRRP protocol as per RFC 3768 does not support millisecond timers, Cisco routers allow you to configure millisecond
timers. You need to manually configure the millisecond timer values
on both the primary and the backup routers. The master advertisement
value displayed in the show vrrp command output on the backup routers
is always 1 second because the packets on the backup routers do not
accept millisecond values.
You must use millisecond timers where absolutely necessary and with
careful consideration and testing. Millisecond values work only under
favorable circumstances, and you must be aware that the use of the
millisecond timer values restricts VRRP operation to Cisco devices