Nortel Networks 450 User 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.
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
, 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