Each MSTI (Multiple Spanning Tree Instances) independently chooses its own root bridge. For example, if two MSTIs are bounded to a region, there is a maximum of two MSTI regional roots and one CIST regional root.
The bridge with the lowest bridge ID becomes the MSTI regional root bridge. The parameters that define the root bridge include the following:
Within an MSTP (Multiple Spanning Tree Protocol) region, the cost from a bridge to the MSTI regional root bridge is known as the MSTI internal path cost. Looking at MSTP region 1 in Closeup of MSTP Region 1, the bridge with ID 60 has a path cost of F to the MSTI regional root bridge.
The MSTI regional root bridge can be the same as or different from the CIST regional root bridge of that region. You achieve this by assigning different priorities to the STP (Spanning Tree Protocol) instances configured as the MSTIs and the CIST. For more information about configuring the bridge ID, see the configure stpd priority command in the ExtremeXOS 22.7 Command Reference Guide.
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