Nortel Networks 450 - Manual (Page 65)

BayStack 450 10/100/1000 Series Switches
302401-D Rev 00 1-39
To connect multiple VLANs across switches with redundant links, you must
disable the STP on all participating switch ports.
Figure 1-23 shows possible
consequences of enabling the STP when using VLANs between untagged
(non-802.1Q tagged) switches.
Figure 1-23. Possible Problems with VLANs and Spanning Tree Protocol
As shown in Figure 1-23 , 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 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
Non-802.1Q tagged switch
Non-802.1Q tagged switch
BS45017A
kombk.book Page 39 Tuesday, June 29, 1999 3:25 PM