Nortel Networks 1000BASE-LX, 1000BASE-SX, 1000ASE-XD, 1000BASE-ZX manual 10006EA

Models: 1000ASE-XD 1000BASE-ZX 1000BASE-SX 1000BASE-LX

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62 Chapter 2 Network configuration

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

Station A

VLAN 1

No

CommunicationsForwarding

VLAN 1

BayStack

420

S1

VLAN 2

Blocking

BayStack

420

S2

VLAN 2

Station B

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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.

209418-A

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Nortel Networks 1000BASE-LX, 1000BASE-SX, 1000ASE-XD, 1000BASE-ZX manual 10006EA