the untagged vlan also called native vlan is an 802.1Q concept that enables a vlan not getting tagged on a 8021.q trunk link for backward compatibility with devices not 802.1Q aware.When a switch receives a frame on a trunk link with no tag it categorizes this frame as part of the native vlan that was configured on the trunk.
Just to add to the good posts from Alain and Alex, Native Vlan is only the Cisco implementation. There is not concept of native vlan from the IEEE standard's perspective. You have options to use a Native vlan which defaults to Vlan 1 or as configured by user, you can also tag all the vlans on the trunk including the native vlan that's being defined to have it compatible with any other vendors switch.
Frames belonging to the native VLAN do NOT carry VLAN tags when sent over the trunk. Conversely, if an untagged frame is received on a trunk port, the frame is associated with the Native VLAN for this port.
According to the 802.1Q standard, the way to identify different vlans in a frame is by adding Vlan ID tag in the frame. This tag identifies what vlan the frame is meant for. If you want multiple vlans to pass via a switch interface, those vlans need to be tagged by configuring a trunk port on a cisco switch.
Access ports are typically untagged, meaning only one vlan passes via the interface. There is also the case of the native vlan. This is a vlan that is not tagged but other vlans that pass via the interface are tagged.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...