ISP redundancy static NAT

[SupportAccount]

Hi guys,

I'm tasked with implementing ISP redundancy in primary/backup at one of our sites running ClusterXL on a pair of 5000 R80.20.

Primary ISP is configured with Static 1:1 NAT for servers on a /27 subnet. Backup ISP is providing a /29 subnet with not enough IP's to static NAT all the servers.

Is there a way that ISP redundancy can be achieved in this? 

Thanks in advance,

[PhoneBoy]

R80.20 is End of Support, just FYI.

Even if you had enough IPs, how would you update the DNS servers so people can connect to the servers when the Backup ISP is engaged?
Do all the servers need to be accessible on the same port (i.e. they're all web servers) or do some of them need to be accessible on different ports?
You might be able to do NAT based on service.
Not sure how that would impact the ISP Redundancy configuration.

[the_rock]

Definitely valid points by phoneboy. PAT maybe?