The invention provides a method for trust relationship detection between a core and access network for a user equipment. The gist is that a security tunnel establishment procedure is used so one entity, be it part of the core network or be it the user equipment itself, is provided with information to determine whether the access network is trusted or untrusted. The information may comprise a first IP address/prefix, which is initially assigned to the user equipment, upon attaching to the access network. The necessary information may further comprise a second IP address/prefix, which is an address/prefix that is allocated at a trusted entity of the core network. Depending which entity determines the trust relationship of the access network, it might be necessary to transmit either the first IP address/prefix or the second IP address/prefix or the first and the second IP address/prefix using the security tunnel establishment procedure.