Allied Telesis AT-GS950/24 - Manual (Page 332)

Appendix A: MSTP Overview
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Multiple Spanning Tree Instance (MSTI)
The individual spanning trees in MSTP are referred to as Multiple Spanning
Tree Instances (MSTIs). A MSTI can span any number of
AT-GS950 switches. The switch can support up to 31 MSTIs at a time.
Before creating a MSTI, you first enable MSTP. Then you must assign the
MSTI a unique number, referred to as the MSTI ID. The range is 1 to 31. After
you have selected an MSTI ID, you need to define the scope of the MSTI by
assigning one or more VLANs to it. An instance can contain any number of
VLANs, but a VLAN can belong to only one MSTI at a time.
Resolving VLAN
Fragmentation
Following are several examples of how MSTP can be applied.
Figure 118 illustrates two AT-GS950/24 switches, each containing the two
VLANs Sales and Production. The ports of each VLAN on each switch are
connected with a direct link using untagged ports. If the switches were running
STP or RSTP, one of these two 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 Figure 118, 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 118. VLAN Fragmentation with STP or RSTP