BayStack User Guide

BayStack 350 10/100/1000 Series Switches

304376-B Rev 00

1-25

To connect multiple VLANs across switches with redundant links, the STP must

be disabled on all participating switch ports. Figure 1-16

shows possible

consequences of enabling the STP when using VLANs between untagged

(non-802.1Q tagged) switches.

Figure 1-16. Possible Problems with VLANs and Spanning Tree Protocol

As shown in Figure 1-16, with STP enabled, only one connection between switch

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.

303303

303

VLAN 1 VLAN 2

Blocking

Station A

Communications

Station B

Forwarding

BayStack 303 switch

BS35017A

kombk.book Page 25 Thursday, February 18, 1999 10:59 AM