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Chapter 13 Spanning Tree Protocol
XGS-4526/4528F/4728F User’s Guide
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The following figure shows an example where there are two MST regions. Regions 
1 and 2 have 2 spanning tree instances. 
Figure 59   MSTIs in Different Regions 
13.1.5.4  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 60   MSTP and Legacy RSTP Network Example