502 CHAPTER 13: V LAN M ANAGEMENTnetworks grow and the amount and types of traffic increase, bridgeflooding may create unnecessary traffic problems that can clog theLAN. VLANs increase the efficiency of your network because eachVLAN can be set up to contain only those devices that need tocommunicate with each other. With VLANs, if a bridge port receives apacket with a broadcast, multicast, or unknown destination address,the switch only forwards the packet to other ports in the same VLANas the receiving port.802.1Q VLANsThe IEEE 802.1Q standard for VLANs aims to:■ Define an architecture to logically partition bridged LANs and provideservices to defined user groups, independent of physical location.■ Allow interoperability between multi-vendor equipment.The 802.1Q standard allows each port on your switch to be placed in:■ Any VLAN defined on the switch.■ Several VLANs at the same time using 802.1Q tagging. The standardspecifies a tag format that embeds VLAN membership informationwithin each packet in a 12-bit VLAN ID (VID). This allows traffic frommultiple VLANs to be carried across one physical link.The standard requires that you define the following information abouteach VLAN on your switch before the switch can use it to forward traffic:■ VLAN Name — This is a descriptive name for the VLAN for example,Marketing or Management.■ 802.1Q VLAN ID — This is used to identify the VLAN if you use802.1Q tagging across your network. It can have a value from 1 to4094, however VID 1 is reserved for the default VLAN. See “TheDefault VLAN” on page 504.Port-based VLANsA port-based VLAN is a collection of ports across one or more switchesthat are configured as members of the same VLAN. Devices attached toports within the VLAN collection are members of the VLAN. In general,VLANs on 3Com switches are port-based VLANs and as a result VLANsupport in 3Com Network Director is aimed at port-based VLANs.
