Use the GUA prefix from the main ISP. During failover, retract it and switch to the GUA prefix from the second ISP. Prefix translate any stragglers that don't s

Use the GUA prefix from the main ISP. During failover, retract it and switch to the GUA prefix from the second ISP. Prefix translate any stragglers that don't switch to the new prefix for whatever reason. For active/active you can distribute both prefixes, but you don't get much control over which network clients pick. You can do the same thing here though: NAT only the outbound connections that you specifically want to steer onto the other ISP. This way you avoid most of the problems of NAT.