Beginning with Configmgr Version 1702, clients use boundary groups to find a new software update point. You can add individual software update points to different boundary groups to control which servers a client can find.
More information about boundary groups and its relation with software update point changes in 1702 and 1706 ,please refer TechNet link https://docs.microsoft.com/en-us/sccm/core/servers/deploy/configure/boundary-groups ,it has lot of information than i explain.
Few weeks ago ,i was looking at the boundaries and boundary groups that are configured for my environment with its fallback information (distribution point and software update point).
I find it hard to locate the boundaries that are configured with fallback distribution points and fallback software update point.Incase you want to know the list of boundaries/boundary groups that are configured with fallback options,there is no default report.
I have created SSRS report that will help to pull information from SQL database into nice reporting .
How does the report work ?
Download the report from Technet Gallary , upload the report into Configmgr SSRS reports folder,change the data source and run the report.
When you run the report, it prompt for Site Code since all the boundary groups that you created must have assigned to specific Site Code for site assignment. If you have not assigned the boundary groups to any specific site code, they will not be listed here .
So choose the Site Code (Primary or Secondary) or select All
Boundary groups that are assigned with specific Site Code will be listed with following information.
Boundary Group Name ,Site system Role,Site Code,Fallback Boundary Group,Fallback Site System,Fallback DP,Fallback SUP.