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■ Port VLAN ID
Packets that are tagged (are carrying the 802.1Q VID information) can be transmitted from one 802.1Q compliant network
device to another with the VLAN information intact. This allows 802.1Q VLAN to span network devices (and indeed, the entire
network – if all network devices are 802.1Q compliant).
Every physical port on a switch has a PVID. 802.1Q ports are also assigned a PVID, for use within the switch. If no VLAN are
defined on the switch, all ports are then assigned to a default VLAN with a PVID equal to 1. Untagged packets are assigned
the PVID of the port on which they were received. Forwarding decisions are based upon this PVID, in so far as VLAN are
concerned. Tagged packets are forwarded according to the VID contained within the tag. Tagged packets are also assigned a
PVID, but the PVID is not used to make packet forwarding decisions, the VID is.
Tag-aware switches must keep a table to relate PVID within the switch to VID on the network. The switch will compare the VID
of a packet to be transmitted to the VID of the port that is to transmit the packet. If the two VID are different the switch will drop
the packet. Because of the existence of the PVID for untagged packets and the VID for tagged packets, tag-aware and tag-
unaware network devices can coexist on the same network.
A switch port can have only one PVID, but can have as many VID as the switch has memory in its VLAN table to store them.
Because some devices on a network may be tag-unaware, a decision must be made at each port on a tag-aware device before
packets are transmitted – should the packet to be transmitted have a tag or not? If the transmitting port is connected to a tag-
unaware device, the packet should be untagged. If the transmitting port is connected to a tag-aware device, the packet should
be tagged.
■ Default VLANs
The Switch initially configures one VLAN, VID = 1, called "default." The factory default setting assigns all ports on the Switch
to the "default". As new VLAN are configured in Port-based mode, their respective member ports are removed from the
"default."
■ Assigning Ports to VLANs
Before enabling VLANs for the switch, you must first assign each port to the VLAN group(s) in which it will participate. By
default all ports are assigned to VLAN 1 as untagged ports. Add a port as a tagged port if you want it to carry traffic for one or
more VLANs, and any intermediate network devices or the host at the other end of the connection supports VLANs. Then
assign ports on the other VLAN-aware network devices along the path that will carry this traffic to the same VLAN(s), either
manually or dynamically using GVRP. However, if you want a port on this switch to participate in one or more VLANs, but none
of the intermediate network devices nor the host at the other end of the connection supports VLANs, then you should add this
port to the VLAN as an untagged port.
VLAN-tagged frames can pass through VLAN-aware or VLAN-unaware network interconnection
devices, but the VLAN tags should be stripped off before passing it on to any end-node host that
does not support VLAN tagging.
User's Manual of WGS-5225-8P2S Series
135
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