Allied Telesis AT-S112 Multiple Spanning Tree Instance Msti, Vlan Fragmentation with STP or Rstp

Appendix A: MSTP Overview

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 Following are several examples of how MSTP can be applied.

Fragmentation

Figure 139 illustrates two AT-GS950/16PS 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-GS950/16PS 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.

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