Allied Telesis AT-8100 Series - Manual (Page 858)

Chapter 58: Multiple Spanning Tree Protocol (MSTP)
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Figure 163 on page 858 illustrates the same two AT-8100 Switches and
the same two virtual LANs. But in this example, the two switches are
running MSTP, and the two VLANs have been assigned different spanning
tree instances. Now that they reside in different MSTIs, both links remain
active, enabling the VLANs to forward traffic over their respective direct
link.
Figure 163. MSTP Example of Two Spanning Tree Instances
An MSTI can contain more than one VLAN. This is illustrated in Figure 164
where there are two AT-8100 Switches with four VLANs. There are two
MSTIs, each containing two VLANs. MSTI 1 contains the Sales and
Presales VLANs and MSTI 2 contains the Design and Engineering
VLANs.