sure all you have to do is use the core rtr as a ntp server (master) and peer the rest of the routers with it
on the core 72xx
Our core router 3750 (192.168.1.1)
confi t
ntp source FastEthernet0/0
ntp master 2 ***
ntp server 192.168.252.1 source FastEthernet0/0 prefer
***(2 is the stratum value from the atomic clock, since i am actual drawing the time from a netware server@192.168.252.1 inhouse (and dishing it out to other routers) i am using 2 instead of 1, if the rtr had it's own bios clock i'd choase 1 for the stratum value)
a branch router (2651XM)
ntp server 192.168.1.1 source FastEthernet0/0 prefer
i am not including all the fancy authentication stuff just to give you the meat and potatoes of what needs to be done.
good luck
-gC-
on branch router
XXXXXXXX#sho clock detail
09:50:07.355 CST Thu Mar 25 2004
Time source is NTP
XXXXXXX#
<pre>
Time source is NTP
XXXXXXXX#sho ntp assoc
address ref clock st when poll reach delay offset disp
*~192.168.1.1 127.127.7.1 2 192 512 377 1.8 0.05 0.0
* master (synced), # master (unsynced), + selected, - candidate, ~ configured
XXXXXX#
</pre>