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CNet Technology Inc.
www.CNet.com.tw
3.3.5
VLAN
Virtual LAN (VLAN) is a group of devices on one or more LANs that are configured so that
they can communicate as if they were attached to the same wire, when in fact they are
located on a number of different LAN segments. Because VLANs are based on logical instead
of physical connections, it is very flexible for user/host management, bandwidth allocation
and resource optimization.
1.Port-Based VLAN: each physical switch port is configured with an access list specifying
membership in a set of VLANs.
2.ATM VLAN - using LAN Emulation (LANE) protocol to map Ethernet packets into ATM
cells and deliver them to their destination by converting an Ethernet MAC address into
an ATM address.
The key for the IEEE 802.1Q to perform the above functions is in its tags. 802.1Q-compliant
switch ports can be configured to transmit tagged or untagged frames. A tag field containing
VLAN (and/or 802.1p priority) information can be inserted into an Ethernet frame. If a port
has an 802.1Q-compliant device attached (such as another switch), these tagged frames can
carry VLAN membership information between switches, thus letting a VLAN span multiple
switches. However, it is important to ensure ports with non-802.1Q-compliant devices
attached are configured to transmit untagged frames. Many NICs for PCs and printers are
not 802.1Q-compliant. If they receive a tagged frame, they will not understand the VLAN tag
and will drop the frame. Also, the maximum legal Ethernet frame size for tagged frames was
increased in 802.1Q (and its companion, 802.3ac) from 1,518 to 1,522 bytes. This could
cause network interface cards and older switches to drop tagged frames as "oversized."
