Hi Carl,
Yes Native vlan is used for untagged traffic.
Now you have to do on all the switches or not it depends on your network requirement.
But if you are doing on one switch trunk port then you should do it on the connected interface of another switchport or else you will get Native vlan mismatch error and communication will stop.
To change the native vlan on IOS switch issue this command
"switchport trunk native vlan "
on CATOS switch
"set vlan mod/port"
once you issue this command on CATOS switch on which trunk port is configured the vlan you mentiones in this command will become native vlan.
HTH
Ankur
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