irule to return html data when all members are down?

So i am wanting to have irule so that when all the pool members are down the following content will be returned, just not sure how to trigger this response when all members are down.

 

 

 

when HTTP_REQUEST {

 

HTTP::respond 200 content {

 

 

 

Site Unavailable

 

 

 

 

 

 

 

 

 

Unavailable

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The service is currently not accessible.

 

 

 

System will be available shortly.

 

 

 

 

 

 

 

}

 

}