Allied Telesis AT-S60 - Manual (Page 260)

Chapter 16: Multiple Spanning Tree
Section II: Advanced Features 260
If the switches were running STP or RSTP, one of the links would be
blocked because the links constitute a physical loop. Which link would
be blocked depends on the STP or RSTP bridge settings. In the example,
the link between the two parts of the Production VLAN is blocked,
resulting in a loss of communications between the two parts of the
Production VLAN.
Figure 84 VLAN Fragmentation with STP or RSTP
AT-8400 SERIES
AT-8400 SERIES
Sales
VLAN
Production
VLAN
Untagged
Ports
Untagged
Ports
Blocked
Port