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Chapter 54 Spanning Tree Protocol
XGS2220 Series User’s Guide
408
• VLAN-to-MST Instance mapping
54.11.3 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.
Therefore 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 two
spanning tree instances.
Figure 315 MSTIs in Different Regions
54.11.4 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.
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