Port priority (lowest number has highest priority) determines the order in which the node will attempt to associate its ports to a spanning tree. If all the ports are assigned the same priority (default: all = 128) then node will prioritize as ETH1, ETH2, ... ETH4. This in order of ascending value of MAC addresses.
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In the WebUI, click the Settings tab.
In the left navigation pane, go to Node Interfaces > Bridge > MSTP > MST Instances.