I see others already did a pretty good job at explaining this (upvotes given as credit). I'll just summarize bits and pieces into one answer and elaborate.
As you seem familiar with Catalyst NS, I'll use this as point of reference. In Catalyst switches, your default VLAN is VLAN 1 which is also the default
Native VLAN
. In a dot1q Ethernet link, just one VLAN can be untagged, otherwise the switch that receives a frame wouldn't be able to tell to which VLAN the received frame belongs to. So in case of Cisco Catalyst switches, all VLAN 1 traffic is untagged by default for any dot1q links you create. It's also a common security practice to change native VLAN to something other than VLAN 1.
In a Catalyst NS, you can change your native (aka untagged) VLAN with
switchport trunk native vlan
command (if-config). So if you type in
switchport trunk native vlan 10
, your VLAN10 traffic on that dot1q link would become untagged, and VLAN 1 traffic would become tagged as a result. In case of BigIP LTM, moving VLAN 10 to untagged is the exact same thing.
Untagged VLAN
is
Native VLAN
.
As you are familiarizing yourself with BigIP L2 terminology, keep in mind that F5 has a tendency to vandalize commonly accepted networking terminology. I.e., "Trunk" term here means something completely different. It is used to describe
Link Aggregation
(known as EtherChannel in Cisco world).