Native VLAN
The VLAN on an 802.1Q trunk whose traffic is sent untagged. Everything else is marked with a VLAN tag; the native VLAN goes across the trunk bare. A common source of both misconfiguration and security holes.
Background: on a trunk you carry several VLANs, each marked with an 802.1Q tag — except the native VLAN, which for historical reasons is sent without a tag (for compatibility with devices that don't understand tags). Risks: both ends of the trunk MUST agree on which VLAN is native (otherwise a "native VLAN mismatch" → traffic leaks between VLANs). Security: VLAN hopping via "double tagging" exploits the native VLAN → best practice is not to use VLAN 1 as native, to set native to an unused "junk VLAN", or to tag everything (tag native). A small detail choice with big security consequences. Related to VLAN hopping and double tagging.