2.1.d [iii] Native VLAN
A frame in the VLAN-aware portion of the network that does not contain a VLAN tag is assumed to be flowing on the native (or default) VLAN. By default , the switch forwards untagged traffic in the native VLAN configured for the port. The native VLAN is VLAN 1 by default.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1628-1629). . Kindle Edition.