Netgear M6100 – Campus Edge and SMB Core Chassis Switches ユーザーガイド

As there are Multiple Instances of Spanning Tree, there is a MSTP state maintained on a 

per-port, per-instance basis (or on a per port per VLAN basis: as any VLAN can be in one 
and only one MSTI or CIST). For example, port A can be forwarding for instance 1 while 
discarding for instance 2. The port states have changed since IEEE 802.1D specification.

To support multiple spanning trees, a MSTP bridge has to be configured with an 
unambiguous assignment of VLAN IDs (VIDs) to spanning trees. This is achieved by: 

Ensuring that the allocation of VIDs to FIDs is unambiguous. 

Ensuring that each FID supported by the Bridge is allocated to exactly one Spanning Tree 

Instance.

The combination of VID to FID and then FID to MSTI allocation defines a mapping of VIDs to 
spanning tree instances, represented by the MST Configuration Table.

With this allocation we ensure that every VLAN is assigned to one and only one MSTI. The 
CIST is also an instance of spanning tree with a MSTID of 0.

An instance may occur that has no VIDs allocated to it, but every VLAN must be allocated to 

one of the other instances of spanning tree. 

The portion of the active topology of the network that connects any two bridges in the same 
MST Region traverses only MST bridges and LANs in that region, and never Bridges of any 
kind outside the Region, in other words connectivity within the region is independent of 
external connectivity.

This example shows how to create an MSTP instance from the M6100 switch. The example 
network has three different M6100 Chassis switch that serve different locations in the 
network. In this example, ports 1/0/1-1/0/5 are connected to host stations, so those links are 
not subject to network loops. Ports 1/0/6 - 1/0/8 are connected across switches 1, 2 and 3.