Untagged and Tagg port

If you want incoming untagged frames on port 1/0/9 associated to VLAN 73, define the PVID for this port to 73, and ensure no other VLANs are marked as [U]ntagged -  or define the port as an Access Port only.

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yivit wrote:

I need to add a new vlan 73.

First define the VLAN 73.

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yivit wrote:

I'm trying to T port 9 on my switch, but it's already T for VLANs 71 and 72 and Untagged for VLAN 1.

When I try to apply the change the config doesn't load and port 9 remains untagged for VLAN 73.

Appears there is some basic understanding problem. When you set the port 1/0/9 to VLAN 73 [T]agged you allow tagged ingress frames for this VLAN, and frames from the other ports/VLAN 73 can egress on this port.

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yivit wrote:

However, when I go to the PVID section I can see that port 9 is tagged on vlan 73.

The PVID does specify the VLAN where untagged ingress frames on this port are assigned to. Tags are applied on egress where defined. 

Historically, all Netgear managed switches had to be configured with VLAN-[U]ntagged and PVID, the ability for defining trunk ports and access ports was added later. So you can deal with VLAN-[U]ntagged and PVID or deal as Access Port. Similar for Trunk ports dealing with the [T]agged marking.