In CLI: slb policy backup <virtual_name> <group_name>

<virtual_name> The name of the virtual service.

<group_name> The specified group name. Allows the user to set the backup policy for a virtual service. You may only have one backup policy per virtual service. Group assigned to the virtual service using backup policy will be used only if there was at least one successful match in a prior Policy, but all Real Services in all matches were down.