VLAN hopping attack.
VLAN hopping (virtual local area network hopping) is a method of attacking a network by sending packets to a port that is not normally accessible from a given end system. (A VLAN is a local area network with a definition that maps devices on some other basis than geographic location - for example, by department, type of user, or primary application.) A VLAN hopping attack can occur in either of two ways. If a network switch is set for autotrunking, the attacker turns it into a switch that appears as if it has a constant need to trunk (that is, to access all the VLAN allowed on the trunk port).