Chapter 11 Spanning Tree Protocol
ES-2108 Series User’s Guide
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• VLAN-to-MST Instance mapping
11.1.4.3 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 51 MSTIs in Different Regions
11.1.4.4 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 52 MSTP and Legacy RSTP Network Example
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