Rainer pls read 
http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960x/software/15-0_2_EX/security/configur...
section 
Multi-auth Per User VLAN assignment
and sentence: "When a hub is connected to an access port, and the port is configured    with an access VLAN (V0).       The host (H1) is assigned to VLAN (V1) through the hub. The operational    VLAN of the port is changed to V1. This behaviour is similar on a single-host    or multi-domain-auth port.    
   When a second host (H2) is connected and gets assigned to VLAN ( V2),    the port will have two operational VLANs (V1 and V2). If H1 and H2 sends    untagged ingress traffic, H1 traffic is mapped to VLAN (V1) and H2 traffic to    VLAN (V2), all egress traffic going out of the port on VLAN (V1) and VLAN (V2)    are untagged."