Details of BGPDNS
In BGPDNS multi-homing, the server operator has upstream links to more than one ISP
From each of these ISP’s the operator also receives a reasonable IP prefix out of their aggregates
One IP address of each prefix of the ISP’s is assigned to the multi-homed server
The router connecting to all these ISP’s does policy routing to direct the outgoing packets to the ISP where the prefix belongs to.