ISIS active and standby routes

Hi,

As the following figure, AR34 is an IS-IS Level-1 router, and AR30 and AR31 are IS-IS Level-1-2 routers. Two IS-IS default routes are generated on AR34. How can I configure the two default routes to work in active/standby mode? In normal cases, only the route to AR30 is available, and the route to AR31 functions as the standby route.

<AR34>dis ip routing-table 

Route Flags: R - relay, D - download to fib

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Routing Tables: Public

         Destinations : 16       Routes : 17       

Destination/Mask    Proto   Pre  Cost      Flags NextHop         Interface

        0.0.0.0/0   ISIS-L1 15   10          D   10.5.14.31      GigabitEthernet0/0/1

                         ISIS-L1 15   10          D   10.5.34.30      GigabitEthernet0/0/1