How to have NTP pool priority set up so that ntp points to a different pool of ntp servers if connectivity to one pool fails?

Issue

• We have 6 NTP server we would like to configure on our NTP service :-

1. ntp0.foo.net
2. ntp1.foo.net
3. ntp2.foo.net

 4. ntp0.bar.org
 5. ntp1.bar.org
 6. ntp2.bar.org

We would like to do the following :

Use as primary sources the following ntp serves:

 1. ntp0.foo.net
 2. ntp1.foo.net
 3. ntp2.foo.net

In case there is any failure use the below servers :

 4. ntp0.bar.org
 5. ntp1.bar.org
 6. ntp2.bar.org