Load Balancing outbound traffic based on SRC IP and having a third link which is backup of the first one

Hello guys:

Here I have a sort of puzzle I would like you to help me to solve.

The TMOS version that I am using is 14.1.0.5. I will load balance outbound traffic by using two ISPs (ISP1 and ISP2). ISP1 has a standby link called ISP1_prime which takes control if only if ISP1 goes down. Additionally, my boss wants to send the networks 172.16.0.0/16, 192.168.2.0/24 and 172.16.3.0/24 via ISP1 and wants to have ISP2 as the backup. On the other hand, he wants to send the nets 10.1.2.0/24 and 10.0.0.0/8 via the ISP2 and having ISP1 as the backup. In this configuration, the ISP1_prime needs to take over ISP1 if the latter goew down. To address this, I have done:

1. Created a pool pool_navigation_nets_users with ISP1 router (priority 5) and ISP2 router (priority 10) to load balance the nets 172.16.0.0/16, 192.168.2.0/24 and 172.16.3.0/24. Priority group activation set to less than 1.
2. Created a pool pool_navigation_nets_servers with ISP1 router (priority 10) and ISP2 router (priority 5) to load balance the nets 10.1.2.0/24 and 10.0.0.0/8. Priority group activation set to less than 1.
3. Configured a virtual server 0.0.0.0/0 and linked to it the following iRule.
4. Configured an iRule like this:

when CLIENT_ACCEPTED { 

   if {[IP::addr [IP::client_addr] 172.16.0.0/16] or [IP::addr [IP::client_addr] 192.168.2.0/24] or [IP::addr [IP::client_addr] 172.16.3.0/24] } {

     pool pool_navigation_nets_users

snat automap } 

   if {[IP::addr [IP::client_addr] 10.0.0.0/8] or [IP::addr [IP::client_addr] 10.1.2.0/24] } {

     pool pool_navigation_nets_servers

snat automap }

}

Such configuration would work if I did not have the ISP1_prime link. I cannot imagine a way to add this third standby link to my configuration. Let´s remember that ISP1_prime takes over iff ISP1 goes down. Any suggestion is appreciated.

Thanks in advance.

Omar