Association between VRRP and NQA does not work on AR router

Customer configures Association between VRRP and NQA. But find it does not work.
Configuration:
interface XYZ
description << VLAN >>
ip address X.X.X.X 255.255.255.0
vrrp vrid 1 virtual-ip X.X.X.X
vrrp vrid 1 priority 120
vrrp vrid 1 track nqa sla icmp_sla reduced 50
#
interface ABC
description << WAN >>
ip address 1.1.1.1 255.255.255.252
#
ip route-static 0.0.0.0 0.0.0.0  Y.Y.Y.Y
#
nqa test-instance sla icmp_sla
test-type icmp
destination-address ipv4 Y.Y.Y.Y
frequency 3
start now

Use the command "display vrrp" and find nqa state is no result
........................
    Config type : normal-vrrp
    Backup-forward : disabled
    Track NQA : sla  icmp_sla   Priority reduced : 50
    NQA state : no result
    Create time : 2013-12-31 19:47:36
    Last change time : 2013-12-31 19:52:57
....................

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1. Check the output of command "display nqa result". Get "no result"
2. Check the meaning of "no result". It means NQA instance is running and did not get result.
3. Check the configuration and found the frequency is too small
• The interval value must be larger than the timeout value.
• The frequency value must be larger than the product of interval (default 4s) and probe-count (default 3 times).
4. Let customer change the frequency to larger value (13s) and it works fine.

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