Allied Telesis AT-S106 - Manual (Page 152)

Chapter 13: Spanning Tree and Rapid Spanning Tree Protocols
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Spanning Tree and VLANs
The spanning tree implementation in the AT-S106 Management software
is a single-instance spanning tree. The AT-GS950/48 Gigabit Ethernet
Smart Switch both support just one spanning tree. You cannot define
multiple spanning trees on either switch.
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 can pose a problem in networks containing multiple
VLANs that span two bridges and are connected with untagged ports. In
this situation, spanning tree blocks a data link because it detects a
suspected data loop. This can cause fragmentation of your VLANs.
This issue is illustrated in Figure 42. VLANs 1 – 3 span two switches. One
link consisting of untagged ports connect each VLAN. If STP or RSTP is
activated on the switches, two of the links are disabled. As a direct result,
two VLANs are disconnected between the bridges. In this example, the
ports (on the non-root switch) that link the two parts of the VLANs 2 - 3 are
changed to the blocking state, which disrupts these VLAN connections.
Figure 46. STP and VLAN Fragmentation with Untagged Ports
VLAN1 VLAN3 VLAN2
Blocked Data Links
Ports Blocked by STP U U U
U U U
VLAN1 VLAN3 VLAN2