3Com SuperStack 3 3812 Implementation Manual

Contents

48 CHAPTER 4: USING R ESILIENCE F EATURESUsing STP on aNetwork withMultiple VLANsThe IEEE Std 802.1D, 1998 Edition does not take into account VLANswhen it calculates STP information — the calculations are only performedon the basis of physical connections. For this reason, some networkconfigurations can result in VLANs being subdivided into a number ofisolated sections by the STP system. Therefore, you must ensure that anyVLAN configuration on your network takes into account the expected STPtopology and alternative topologies that may result from link failures.For example, Figure 11 shows a network containing VLANs 1 and 2. Theyare connected using the 802.1Q-tagged link between Switch B andSwitch C. By default, this link has a path cost of 100 and is automaticallyblocked because the other Switch-to-Switch connections have a path costof 36 (18+18). This means that both VLANs are now subdivided — VLAN1 on Switch units A and B cannot communicate with VLAN 1 on SwitchC, and VLAN 2 on Switch units A and C cannot communicate with VLAN2 on Switch B.Figure 11 Configuration that separates VLANsTo avoid any VLAN subdivision, it is recommended that all inter-Switchconnections are made members of all available 802.1Q VLANs to ensureconnectivity at all times. For example, the connections between SwitchesA and B, and between Switches A and C should be 802.1Q tagged andcarrying VLANs 1 and 2 to ensure connectivity.For more information about VLAN Tagging, see Chapter 8 “Setting UpVirtual LANs”.Switch ASwitch B Switch C100BASE-TXfull duplex linkcarrying VLAN 1 only(path cost 18)100BASE-TXfull duplex linkcarrying VLAN 2 only(path cost 18)802.1Q-tagged,10BASE-T half duplex linkcarrying VLANs 1 and 2(path cost 100)Block