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Figure 51 mstis in different regions, Figure 52 mstp and legacy rstp network example – ZyXEL Communications ES-2108 Series User Manual

Page 105

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Chapter 11 Spanning Tree Protocol

ES-2108 Series User’s Guide

105

• 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