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Chapter 54 Spanning Tree Protocol
XGS2220 Series User’s Guide
407
54.11 Technical Reference
This section provides technical background information on the topics discussed in this chapter.
54.11.1 MSTP Network Example
The following figure shows a network example where two VLANs are configured on the two switches. If
the switches are using STP or RSTP, the link for VLAN 2 will be blocked as STP and RSTP allow only one link
in the network and block the redundant link.
Figure 313 STP/RSTP Network Example
With MSTP, VLANs 1 and 2 are mapped to different spanning trees in the network. Therefore traffic from
the two VLANs travel on different paths. The following figure shows the network example using MSTP.
Figure 314 MSTP Network Example
54.11.2 MST Region
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
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