Sharepoint 2010 custom navigation provider

Created a custom navigation provider using this link

http://code.msdn.microsoft.com/office/SharePoint-2010-Developing-5f0e73c2

but it retrieves all nodes from the root web in site collection so I had to configure the SiteMapDataSource object to show nodes only from current site as below

<asp:SiteMapDataSource ShowStartingNode="true" SiteMapProvider="MyCustomSiteMapProvider"

                                                ID="topSiteMap" runat="server" StartFromCurrentNode="true" StartingNodeOffset="0" />