untagPVIDonly - on a nortel switch

The VLAN tag information is referred to as VLAN ID (VID). Ports on a switch are configured as being members of a VLAN designated by the VID for that port. The default VID for a port is referred to as the Port VID (PVID). The VID can be added to an Ethernet packet either by a VLAN-aware host, or by the switch in the case of VLAN-unaware hosts. Ports on an Ethernet switch must therefore be configured with information indicating whether the host connected is VLAN-aware.

For VLAN-unaware hosts, a port is set up as untagged and the switch will tag all packets entering through that port with the Port VLAN ID (PVID). It will also untag all packets exiting that port before delivery to the VLAN unaware host. A port used to connect VLAN-unaware hosts is called an untagged port, and it can be a member of only a single VLAN identified by its PVID. Hosts that are VLAN-aware can insert and remove their own tags and can be members of more than one VLAN. These hosts are typically attached to ports that do not remove the tags before delivering the packets to the host, but will insert the PVID tag when an untagged packet enters the port. A port will only allow packets that are untagged or tagged with the tag of one of the VLANs that the port belongs to. These VLAN rules are in addition to the regular media access control (MAC) address-based forwarding rules followed by a switch. Therefore, a packet with a broadcast or multicast destination MAC is also delivered to member ports that belong to the VLAN that is identified by the tags in the packet. This mechanism ensures the logical separation of the physical network based on membership in a VLAN.

Ok Im sort of getting there I think - What I have is two switchs (the connecting ports are both members of 2 VLANS) a few ports on each switch are members of either/or vlan. The setting on the 2 connecting ports/uplink ports is set to UntagPVIDonly. The other option which i thought it should be is a trunk port - (i thought that is what a trunk is  - a port with multiple VLANS) - Im still not quite getting why you would set these uplink ports to untagPVIDonly and not trunk? - Sorry I didnt set this up - sorry if im missing concept still - im just trying to understand - so these two ports that connect the two switchs are VLAN aware?  - why the need to  remove the PVID tag.

I think the main question is -  Are there any hosts on your network that are VLAN-unaware? if not then you don't need to untag and you could just trunk the traffic.  If you have hosts that are VLAN-unaware then you need to untag so that the traffic gets trunked to the right host and the VLANID will be added if needed .  If you untag the trunk port it still trunks traffic

Sorry please bare with me. The devices attached are standard PC's and a few linux servers. I think then they are VLAN unaware correct? - So if they are VLAN unaware your saying you need to remove the tag(PVID on the switch ports) so the traffic will get to the right VLAN - im still confused sorry :( wouldnt removing the tag make the traffic NOT know which VLAN it belongs to as the ID has been removed/untagged?  I know im likely misunderstanding again sorry.

Thanks I think i can safely change to trunk then - nothing unusual -its working with untagpvid so my rule is it aint broke so i wont b fixing.

So I see read a por just t specified as trunk DOES attach to each frame the PVID - and as the hosts are VLAN aware they can read the frame and understand the PVID on the frame.   If the hosts were VLAN UNaware and the PVID was tagged on the frame the host would have trouble with processing the frame as it would not understand what he PVID tag on the frame meant - Am I correct with this understanding - Please provide answer and points are yours. Thanks with help understanding been great

The untagPVIDonly setting is normally used for ports that have a VOIP phone with a PC connected through the phone and using same port configured with a voice VLAN for the phone and a data VLAN for the PC. The phone's packets are tagged and assigned to the VLAN/subnet for the phones while the PC's packets would be untagged. The untagPVIDonly setting is what allows the PC's untagged packets to get assigned to the data VLAN.