ZyXEL Communications VES1724-56 User Manual page 164

Chapter 14 Spanning Tree Protocol
Devices that belong to the same MST region are configured to have the same MSTP configuration
identification settings. These include the following parameters:
• Name of the MST region
• Revision level as the unique number for the MST region
• VLAN-to-MST Instance mapping
14.1.5.3 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 95 MSTIs in Different Regions
14.1.5.4 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 96 MSTP and Legacy RSTP Network Example
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VES1724-56 User's Guide