Hi,
Native VLAN:VLAN that is not associated explicitly to any tag on an 802.1Q link.
Reference: http://www.cisco.com/en/US/products/hw/switches/ps708/products_white_paper09186a008013159f.shtml
In other words: when yo have a link that is carring multiple VLAN with 802.1q, you can have a VLAN that is not tagged, that is the VLAN is not identified by a VID. This VLAN is called "native VLAN".
I hope this helps.
Best regards.
Massimiliano.
