Allied Telesis AT-9000 Series - Manual (Page 672)

Chapter 45: Multiple Spanning Tree Protocol
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Figure 123. MSTP Example of Two Spanning Tree Instances
An MSTI can contain more than one VLAN. This is illustrated in Figure 124
where there are two AT-9000 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.
Figure 124. Multiple VLANs in an MSTI
In this example, because an MSTI contains more than one VLAN, the links
between the VLAN parts are made with tagged, not untagged, ports so
that they can carry traffic from more than one virtual LAN. Referring again
to Figure 124, the tagged link in MSTI 1 is carrying traffic for both the
Presales and Sales VLANs while the tagged link in MSTI 2 is carrying
traffic for the Design and Engineering VLANs.