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9032947 Virtual LANs (VLANs) 83
APPENDIX D. VIRTUAL LANS (VLANS)
VLANs and Frame Tagging
The ELS100-24TXG supports IEEE 802.1Q-compliant virtual LANs
(VLANs). This capability provides a highly efficient architecture for
establishing VLANs within a network and for controlling broadcast/
multicast traffic between workgroups. Central to this capability is an
explicit frame tagging approach for carrying VLAN information between
interconnected network devices.
With frame tagging, a four byte data tag field is appended to frames that
cross the network. The tag identifies which VLAN the frame belongs to.
The tag may be added to the frame by the end station itself or by a
network device, such as a switch. In addition to VLAN information, the
relative priority of the frame in the network can specified by the tag (see
Appendix E, “Class of Service”).
In the example in Figure D-1 below, the workstations attached to the top
switch are members of the same two VLANs as the workstations attached
to the bottom switch. The 802.1Q tags are used to carry VLAN information
in the frames traveling between the two switches.
Figure D-1. Two Switch VLAN Configuration
If the ELS100-24TXG is so configured, and, if the frame does not already
contain a tag, a VLAN identifier is placed in a tag which is appended to
frames as they are received by the switch. Within the switch and between
switches compliant with IEEE 802.1Q, these identifiers are used to control
the destination of broadcast frames, preventing them from being flooded