I suppose that you are talkign about the "Vertical" - Simple mode of the NavigationControl. In this mode we use a control that inherits from RadTabStrip. The NavigationControl is BaseDataBoundControl. The control is automatically populated by SiteMapDataSource and the levels you see depend on the pages structure you have.
The control has several public properties that you can set from the "Advanced" tab of the control
1.ShowExpanded - shows all items expanded when possible
2.LevelsToExpand - how many levels should be expanded
3.NavigationAction - show the item OnClick
Inside TabDataBound event of the control we get the current tab and set it as selected using server side code.
All other properties, methods etc are exposed by RadTabStrip
control and you can use them in custom implementations.
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