VIAVI Test Center | How do we setup MST with the VIAVI device both CIST 0 and MSTI 1 root bridge?

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You can't, for Multiple Spanning Tree (MST) when you have a instance (MSTI) that acts as a Common and Internal Spanning Tree (CIST) to become the Root Bridge they need to be the Multiple Spanning tree Instance number 0 (MSTI 0), if you choose another instance number they wont become the root bridge. you can see this extract from the IEEE 802.1Q-2018. Clause 13.4.3.1 (MSTP operation): “Each Bridge operating MSTP shall have an Internal Spanning Tree (IST), which is MSTI 0, and may have one or more additional MST Instances (MSTIs), numbered from 1 upwards.”“The IST always participates in the Common and Internal Spanning Tree (CIST), which spans the entire Bridged LAN.” Clause 13.5.5.2 (CIST Root Bridge selection): “The CIST Root is selected using standard spanning tree procedures (as defined in IEEE 802.1D), but only using MSTI 0 BPDUs. MSTIs 1 and above do not participate in this election.”