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Using the BayStack 350 10/100/1000 Series Switch
1-28
309979-A Rev 00
To connect multiple VLANs across switches with redundant links, the STP must
be disabled on all participating switch ports. Figure 1-18
shows possible
consequences of enabling the STP when using VLANs between untagged
(non-802.1Q tagged) switches.
Figure 1-18. Possible Problems with VLANs and Spanning Tree Protocol
As shown in Figure 1-18, with STP enabled, only one connection between S1 and
S2 is forwarding at any time. Communications failure occurs between VLAN 2 of
S1 and VLAN 2 of S2, blocking communications between Stations A and B.
The link connecting VLAN 1 on switches S1 and S2 is selected as the forwarding
link based on port speed, duplex mode, and port priority. Because the other link
connecting VLAN 2 is placed into Blocking mode, stations on VLAN 2 in switch
S1 cannot communicate with stations in VLAN 2 on switch S2. With multiple
links only one link will be forwarding.
S2
S1
VLAN 1 VLAN 2
VLAN 1 VLAN 2
Blocking
Station A
No
Communications
Station B
Forwarding
Non-802.1Q
tagged switch
BS35017A
Non-802.1Q
tagged switch