Hello David,
Can you try specifying the stp root via the primary/secondary commands - spanning-tree vlan xxx root primary/secondary. instead of specifying the priority value, this will decrease its own priority value if another switch challange's for stp root with it present priority vlaue.
Also you could apply spanning-tree rootguardto protect your root switch designated ports becoming root ports towards a new stp root switch
To influence a directly connected downstream switch from the root to elect its stp root port use - spanning-tree vlan x port-priority x ( highest value less preferred)
To influence a local switch towards the root to elect its stp root port use - spanning-tree vlan x cost y ( highest value less preferred)
Welcome any other CSCr's to jump in here and correct me if i am wrong?
res
Paul
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Kind Regards
Paul