Chapter 13 Spanning Tree Protocol
GS2220 Series User’s Guide
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With MSTP, VLANs 1 and 2 are mapped to different spanning trees in the network. Thus traffic from the
two VLANs travel on different paths. The following figure shows the network example using MSTP.
Fig ure 142 MSTP Network Example
13.10.2 MST Re g io n
An MST region is a logical grouping of multiple network devices that appears as a single device to the
rest of the network. Each MSTP-enabled device can only belong to one MST region. When BPDUs enter
an MST region, external path cost (of paths outside this region) is increased by one. Internal path cost (of
paths within this region) is increased by one when BPDUs traverse the region.
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
13.10.3 MST Insta nc e
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 two
spanning tree instances.
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