VLAN hopping
An attack where an attacker on one VLAN manages to send traffic into another VLAN — thereby bypassing the segmentation meant to keep them apart. Two main methods: switch spoofing and double tagging.
Switch spoofing: the attacker's computer pretends to be a switch and negotiates a trunk (via DTP, Dynamic Trunking Protocol) → then gets access to all VLANs on the trunk. Countermeasure: disable DTP, hard-set access ports to "access mode". Double tagging: the attacker puts two 802.1Q tags on the frame; the first (native VLAN) is stripped by the first switch, the second carries the frame on into the target VLAN. Countermeasure: don't use the native VLAN for any real traffic. Win for the attacker: bypass L2 segmentation, reach otherwise-isolated segments. Defense: disable DTP, change away from VLAN 1, dedicate an unused native VLAN. A classic reason VLAN security requires care. Related to double tagging and native VLAN.