ZyXEL Communications ONU-2024 Series, User Manual

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Chapter 11 Spanning Tree Protocol
ONU User’s Guide
99
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
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 41 MSTIs in Different Regions
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 42 MSTP and Legacy RSTP Network Example