ZyXEL Communications XGS-4728F User Manual page 130

Chapter 13 Spanning Tree Protocol
The following figure shows an example where there are two MST regions. Regions
1 and 2 have 2 spanning tree instances.
Figure 66 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 67 MSTP and Legacy RSTP Network Example
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XGS-4728F User's Guide