PLANET FGSW-2840 - Manual (Page 101)

User’s Manual of FGSW-2840 / FGSW-4840S
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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 Managed Switch initially configures one VLAN, VID = 1, called "default." The factory default setting assigns all ports on the
Managed 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 Managed 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 Managed 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.
■ VLAN Classification
When the Managed Switch receives a frame, it classifies the frame in one of two ways. If the frame is untagged, the Managed
Switch assigns the frame to an associated VLAN (based on the default VLAN ID of the receiving port). But if the frame is tagged,
the Managed Switch uses the tagged VLAN ID to identify the port broadcast domain of the frame.
■ Port Overlapping
Port overlapping can be used to allow access to commonly shared network resources among different VLAN groups, such as
file servers or printers. Note that if you implement VLANs which do not overlap, but still need to communicate, you can connect
them by enabled routing on this Managed Switch.
■ Untagged VLANs
Untagged (or static) VLANs are typically used to reduce broadcast traffic and to increase security. A group of network users
assigned to a VLAN form a broadcast domain that is separate from other VLANs configured on the Managed Switch. Packets
are forwarded only between ports that are designated for the same VLAN. Untagged VLANs can be used to manually isolate
user groups or subnets.