Allied-telesis AT-GS950/16 User Manual download pdf (Page 370)

Appendix A: MSTP Overview

370

Multiple Spanning Tree Instance (MSTI)

The individual spanning trees in MSTP are referred to as Multiple Spanning

Tree Instances (MSTIs). An MSTI can span any number of

AT-GS950 switches. The switch can support up to 31 MSTIs at a time.

Before creating an 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 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 154 illustrates two AT-GS950/16 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 154, 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 154. VLAN Fragmentation with STP or RSTP

Figure 155 on page 371 illustrates the same two AT-GS950/16 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.

Blocked Port

Untagged PortsUntagged Ports

Sales VLAN Production VLAN

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Allied-telesis AT-GS950/16 User Manual Page 370