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Chapter 22: Multiple Spanning Tree Protocol
234 Section V: Spanning Tree Protocols
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-9400 Switches. The switch can support up to 16 MSTIs at a time.
To create a MSTI, you first assign it a number, referred to as the MSTI ID.
The range is 1 to 15. (The switch is shipped with a default MSTI with an
MSTI ID of 0. This default spanning tree instance is discussed later in
“Common and Internal Spanning Tree (CIST)” on page 244.)
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.
Following are several examples. Figure 23 illustrates two AT-9400
Switches, each containing the two VLANs Sales and Production. The two
parts of each VLAN are connected with a direct link using untagged ports
on both switches. If the switches were running STP or RSTP, one of the
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 23, 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.