Instruction Manual
53
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Up to 255 VLANs based on the IEEE 802.1Q standard.
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Port overlapping, allowing a port to participate in multiple VLANs.
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End stations can belong to multiple VLANs.
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Passing traffic between VLAN-aware and VLAN-unaware devices.
IEEE 802.1Q Standard
IEEE 802.1Q (tagged) VLAN are implemented on the Switch. 802.1Q VLAN require
tagging, which enables them to span the entire network (assuming all switches
on the network are IEEE 802.1Q-compliant).
VLAN allow a network to be segmented in order to reduce the size of broadcast
domains. All packets entering a VLAN will only be forwarded to the stations (over
IEEE 802.1Q enabled switches) that are members of that VLAN, and this includes
broadcast, multicast and unicast packets from unknown sources.
VLAN can also provide a level of security to your network. IEEE 802.1Q VLAN will
only deliver packets between stations that are members of the VLAN. Any port
can be configured as either tagging or untagging.:
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The untagging feature of IEEE 802.1Q VLAN allows the VLAN to work with
legacy switches that don't recognize VLAN tags in packet headers.
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The tagging feature allows the VLAN to span multiple 802.1Q-compliant
switches through a single physical connection and allows Spanning Tree to be
enabled on all ports and work normally.
Terms:
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Tagging
: The act of putting 802.1Q VLAN information into the header of a
packet.
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Untagging
: The act of stripping 802.1Q VLAN information out of the packet
header.
802.1Q VLAN Tags
There are four additional octets inserted after the source MAC address. Their
presence is indicated by a value of 0x8100 in the EtherType field. When a packet's
EtherType field is equal to 0x8100, the packet carries the IEEE 802.1Q/802.1p tag.
The tag is contained in the following two octets and consists of 3 bits of user
priority, 1 bit of Canonical Format Identifier (CFI - used for encapsulating Token
Ring packets so they can be carried across Ethernet backbones), and 12 bits of
VLAN ID (VID). The 3 bits of user priority are used by 802.1p. The VID is the VLAN
identifier and is used by the 802.1Q standard. Because the VID is 12 bits long,
4094 unique VLAN can be identified.
The tag is inserted into the packet header making the entire packet longer by 4
