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62 Chapter 2 Network configuration
209418-A
To connect multiple VLANs across switches with redundant links, you must
disable the STP on all participating switch ports. Figure 19 shows possible
consequences of enabling the STP when using VLANs between untagged
(non-802.1Q tagged) switches.
Figure 19 Possible problems with VLANs and Spanning Tree Protocol
As shown in Figure 19, with STP enabled, only one connection between Switch
S1 and Switch 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 STP selects the link connecting VLAN 1 on Switches S1 and S2 as the
forwarding link based on port speed, duplex-mode, and port priority. Because the
other link connecting VLAN 2 is in 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
BayStack
420
BayStack
420
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