Cabletron Systems ELS100-24TXG Installation And User Manual

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Cabletron Systems ELS100-24TXG Installation And User Manual Manual pdf 97 page image

9032947 Virtual LANs (VLANs) 83APPENDIX D. VIRTUAL LANS (VLANS)VLANs and Frame TaggingThe ELS100-24TXG supports IEEE 802.1Q-compliant virtual LANs(VLANs). This capability provides a highly efficient architecture forestablishing VLANs within a network and for controlling broadcast/multicast traffic between workgroups. Central to this capability is anexplicit frame tagging approach for carrying VLAN information betweeninterconnected network devices.With frame tagging, a four byte data tag field is appended to frames thatcross 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 anetwork device, such as a switch. In addition to VLAN information, therelative priority of the frame in the network can specified by the tag (seeAppendix E, “Class of Service”).In the example in Figure D-1 below, the workstations attached to the topswitch are members of the same two VLANs as the workstations attachedto the bottom switch. The 802.1Q tags are used to carry VLAN informationin the frames traveling between the two switches.Figure D-1. Two Switch VLAN ConfigurationIf the ELS100-24TXG is so configured, and, if the frame does not alreadycontain a tag, a VLAN identifier is placed in a tag which is appended toframes as they are received by the switch. Within the switch and betweenswitches compliant with IEEE 802.1Q, these identifiers are used to controlthe destination of broadcast frames, preventing them from being flooded