Nortel BayStack 450 Using Manual page 65

To connect multiple VLANs across switches with redundant links, you must
disable the STP on all participating switch ports.
consequences of enabling the STP when using VLANs between untagged
(non-802.1Q tagged) switches.
Communications
Figure 1-23.
As shown in
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.
302401-D Rev 00
Station A
No
Station B
Possible Problems with VLANs and Spanning Tree Protocol
Figure 1-23, with STP enabled, only one connection between S1 and
BayStack 450 10/100/1000 Series Switches
VLAN 1
Forwarding
VLAN 1
Figure 1-23 shows possible
Non-802.1Q tagged switch
S1
VLAN 2
Blocking
Non-802.1Q tagged switch
S2
VLAN 2
BS45017A
1-39