Allied Telesis SwitchBlade x3100 Series Manual page 364

Introduction
4.4 VLAN (802.3)
4.4.1 Introduction
A VLAN is a virtual subnetwork that allows devices to be grouped into one logical broadcast domain. This
allows broadcasts from one VLAN to be sent only to members on the same VLAN.
4.4.2 Virtual LANs (VLANs)
4.4.2.1 VLAN Tagging
An Ethernet packet can contain a VLAN tag, with fields that specify VLAN membership (the VLAN ID or VID)
and user priority. The VLAN tag is described in IEEE Standard 802.3ac, and is four octets that can be inserted
between the Source Address and the Type/Length fields in the Ethernet packet. To accommodate the tag, Stan-
dard 802.3ac also increased the maximum allowable length for an Ethernet frame to 1522 octets (the minimum
size is 64 octets). IEEE Standard 802.1q specifies how the data in the VLAN tag is used to switch frames. VLAN-
aware devices are able to add the VLAN tag to the packet header. VLAN-unaware devices cannot add or read
the VLAN tag.
Ethernet packets which contain a VLAN tag are referred to as tagged frames.
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Switch ports that transmit tagged frames are referred to as tagged ports.
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Ethernet packets which do not contain the VLAN tag are referred to as untagged frames.
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Switch ports that transmit untagged frames are referred to as untagged ports.
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A VLAN can therefore consist of:
A set of untagged ports, in which the ports receive and transmit untagged packets.
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A set of tagged ports, in which all ports for the VLAN transmit tagged frames
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A mixture of tagged and untagged ports, where on some ports the VLAN receives and transmits tagged
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frames and on other ports the VLAN receives and transmits untagged frames.
The SBx3112 accepts VLAN tagged frames, and support the VLAN switching required by these tags. A network
can contain a mixture of VLAN aware devices and VLAN unaware devices (e.g., workstations and legacy
switches that do not support VLAN tagging). The SBx3112 can be configured to send VLAN-tagged or
untagged frames on each port, depending on whether or not the devices connected to the port are VLAN
aware. By assigning a port to two different VLANs (one as an untagged port and another as a tagged port), it is
possible for the port to transmit both VLAN-tagged and untagged frames.
When VLAN membership is determined using VLAN tagging, switch ports and network resources can be used
more efficiently, since a port can belong to several VLANs. Moreover, one port can be used to uplink (trunk) all
VLAN traffic between the SBx3112 and another VLAN-aware switch, since this port can be configured to
include all VLANs on the SBx3112.
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Software Reference for SwitchBlade x3100 Series Switches (Layer Two Switching)
Introduction