IPv6 tuel interface

I don't know its valid question or not.My question is i have eth0 interface with public IPv4 address and i already created 6to4 tuel for ipv6.So my question is when i am trying to access a ipv6 host or address ,the system is automatically find the 6to4 for traffic or we need to specify the particular interface name.Suppose i am trying to access rdesktop over ipv6,we need to specify interface name?