Creating iRule for web proxies with site preference

So if you decide to do the loadbalancing on the LTM then it's simple to change pools based on source address using the iRule as you have created. The source address matching section of that iRule should use the class iRule command to find the address within a datagroup. This is completely separate to the standard topology loadbalancing within GTM. Just to confuse things more, GTM has iRule events and you can use the matchregion and whereis iRule commands.

In general, it is good practise to perform the function where it is simplest and most efficient and using the simplest method - don't write complex iRules if you can use built-in features. If it were me, i'd decide on whether i wanted an explicit or transparent proxy and if explicit then perform GTM loadbalancing across sites, if transparent then perform LTM loadbalancing across links.