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Appendix A: MSTP Overview
352
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 139 illustrates two AT-GS950/48PS 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 139, the link between the two ports of the
Production VLAN is blocked, resulting in a loss of communications between
the two parts of the Production VLAN.
Figure 139. VLAN Fragmentation with STP or RSTP
Figure 140 on page 353 illustrates the same two AT-GS950/48PS 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.