What you are encountering is a normal aspect of configuring backup interface. When you configure a primary interface with backup interface pointing to something like ISDN the ISDN goes into a special standby state and will not activate until there is a failure of the primary. And shutdown does not qualify as a failure. You must actually make the primary go into line protocol down state (not administratively down) for the backup to activate.
This is one of the reasons why many people are using floating static routes or dialer watch for backup purposes instead of using the backup interface concept.
HTH
Rick
HTH
Rick
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