I have a ds3 capable of 28 full channelized T1s. I need to configure one of them for 768k. Would I just configure this to say
t1 1 channel-group 1 timeslots 1-12 instead of t1 1 channel-group 1 timeslots 1-24 on the cpe and pe router? just the pe?
you need to configure on both ends as both parts need to agree on what DS0 timeslots are usable.
on PE side you need to configure the DS3 and the T1, if the CE has a T1 you will configure only that
Be aware that RED or ADM in the middle may not able to multiplex sub T1 / sub E1 so it may be not possible to use B channels 13-24 for another customer (it depends on RED/ADM capabilities)
see for example:
Hope to help