F5 rule for access particular CAS server from Cisco TMSXE server

It might help to give us the current configuration that you've got on the F5 currently, especially the rule that you're talking about.

But the simple answer if you want to have one server take all the traffic unless it is out of service, use priority group on the pool.

It's possible that I misunderstood your question.

So, in this case, since what you're looking for all comes canned within LTM, you can do just what you're looking for with something like the following. In this config, 10.10.10.1 will be the only member taking traffic, unless it is out of service. In that case, and only in that case, 10.10.10.2 will take over.

I've put cookie persistence on for this example, but that should be modified to meet your needs.

ltm pool test-pool {
    members {
        10.10.10.1:http {
            address 10.10.10.1
            priority-group 10
            session monitor-enabled
        }
        10.10.10.2:http {
            address 10.10.10.2
            session monitor-enabled
        }
    }
    min-active-members 1
    monitor http
}
ltm virtual test-vip {
    destination 10.20.10.1:http
    ip-protocol tcp
    mask 255.255.255.255
    persist {
        cookie {
            default yes
        }
    }
    pool test-pool
    profiles {
        http { }
        oneconnect { }
        tcp { }
    }
    source 0.0.0.0/0
    source-address-translation {
        type automap
    }
}