I am trying to setup time service on two devices. Once is a UC520, the second an 1840 router. I would like to use the FQDN of the time server which is north-america.pool.ntp.org. This is the recommended procedure as per ntp.org due to changes of IP addresses of time servers.
On the UC520 this is not a problem. I type in "ntp server north-america.pool.ntp.org" and this is how the command stays in the config. On the 1800 the FQDN is resolved and inserted into the config as an IP address. This works for now but kinda defeats the purpose of using a FQDN.
Please be aware that even if the domain name stays in config, that does NOT mean the router will follow any change in the corresponding A record after it is configured. It will NOT.
So, the whatever IOS is running on the 1800 is doing the right thing, it resolves the domain to IP once and leave the latter in config instead of fooling you in thinking that the router will follow DNS in NTP - again, it will not.
In any case, the difference is due to different IOS version
That's the way NTP works on IOS - it uses addresses only. Be that right or wrong, we can't change it
If you don't like it please don't complain to me - complain to Cisco.