Allied Telesis AT-GS950/8 manual Spanning Tree and VLANs

Spanning Tree and VLANs

AT-S82 Management Software User’s Guide

If you decide to activate spanning tree on the switch, there is no reason not to activate RSTP on an AT-GS950/8 WebSmart switch even when all other switches are running STP. The switch can combine its RSTP with the STP of the other switches. The switch monitors the traffic on each port for BPDU packets. Ports that receive RSTP BPDU packets operates in RSTP mode while ports receiving STP BPDU packets operate in STP mode.

The spanning tree implementation in the AT-S82 management software is

asingle-instance spanning tree. The switch supports just one spanning tree. You cannot define multiple spanning trees.

The single spanning tree encompasses all ports on the switch. If the ports are divided into different VLANs, the spanning tree crosses the VLAN boundaries. This point can pose a problem in networks containing multiple VLANs that span different switches and are connected with untagged ports. In this situation, STP blocks a data link because it detects a data loop. This can cause fragmentation of your VLANs.

This issue is illustrated in Figure 28. Two VLANs, Sales and Production, span two AT-GS950/8 WebSmart switches. Two links consisting of untagged ports connect the separate parts of each VLAN. If STP or RSTP is activated on the switches, one of the links is disabled. In the example, the port on the top switch that links the two parts of the Production VLAN is changed to the block state. This leaves the two parts of the Production VLAN unable to communicate with each other.

Sales	Production
VLAN	VLAN	Blocked Port

		912
		Blocked Data Link
Sales	Production	912
VLAN	VLAN

Figure 28. VLAN Fragmentation

You can avoid this problem by not activating spanning tree or by connecting VLANs using tagged instead of untagged ports. (For

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