http redirect

I guess this depends on your exact requirements, but here's a pretty common use case (no active pool members):

when HTTP_REQUEST {
    if { [active_members http_pool] < 1 } {
        HTTP::redirect "http://dr-site.example.com"
    }
}

when "http_pool" is the name of the pool assigned to the VIP (along with this iRule). If all of the pool members are down, it will simply redirect the user to a new URL.