Chapter 13 Spanning Tree Protocol
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• VLAN-to-MST Instance mapping
13.11.3 MST Instance
An MST Instance (MSTI) is a spanning tree instance. VLANs can be configured to run on a specific
MSTI. Each created MSTI is identified by a unique number (known as an MST ID) known internally
to a region. Thus an MSTI does not span across MST regions.
The following figure shows an example where there are two MST regions. Regions 1 and 2 have 2
spanning tree instances.
Figure 97 MSTIs in Different Regions
13.11.4 Common and Internal Spanning Tree (CIST)
A CIST represents the connectivity of the entire network and it is equivalent to a spanning tree in
an STP/RSTP. The CIST is the default MST instance (MSTID 0). Any VLANs that are not members of
an MST instance are members of the CIST. In an MSTP-enabled network, there is only one CIST
that runs between MST regions and single spanning tree devices. A network may contain multiple
MST regions and other network segments running RSTP.
Figure 98 MSTP and Legacy RSTP Network Example
