I set the PPPoE server with IP-Pool and is working perfectly. I just do not understand that when a customer disconnects and returns to connect later he receives the same IP. That is, the IP address assigned to each customer connection does not change.

Any tips?

yep, all recent addresses are returned to their owners =) if user reconnects after some hours - he receives new address

and there is no way I can set to force a change of IP?