My nas is a mikrotik last version. He sends always the same value as port(ether1 for me). I also use simultaneous-use. The problem is that radutmp make the sessions unique after port. I modified the sources and I replaced nas-port-id with acct-session-id, but acct-session-id is olways increasing.
I hope that in a future version, radius will have a choice for unique sesions(nas-port or username) - List info/subscribe/unsubscribe? See http://www.freeradius.org/list/users.html
