Allied Telesis AT-FS970M Series - Manual (Page 836)

Chapter 56: Multiple Spanning Tree Protocol
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Figure 158 illustrates the same two AT-FS970M 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 158. MSTP Example of Two Spanning Tree Instances
An MSTI can contain more than one VLAN. This is illustrated in Figure 159
on page 837 where there are two AT-FS970M 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.