iRule for load balancing to different virtual server depending on the URI path

Bobow This might work but what you are attempting to do seems a whole lot more complicated this way and can most likely be done cleanly by only matching the URI path to a specific destination VS rather than letting load balancing occur and then changing the HOST and URI path after the fact.

when LB_SELECTED priority 500 {

    set SELECTED_POOL_MEMBER[LB::server addr]

}

when HTTP_REQUEST_SEND priority 500 {

    if { ${SELECTED_POOL_MEMBER} == vs_b_ip } {
        HTTP::uri [string map {"/aa" "/"}[HTTP::uri]]
        HTTP::host [string map {"test.com" "test1.com"}[HTTP::host]]
    } elseif { ${SELECTED_POOL_MEMBER} == vs_c_ip } {
        HTTP::uri [string map {"/aa" "/"}[HTTP::uri]]
        HTTP::host [string map {"test.com" "test1.com"}[HTTP::host]]
    }

}

As many have said before "Just because it can be done with an iRule doesn't mean you should." so you might want to look at alternatives that work a bit better. For instance you might consider allowing the servers to listen for the orignal host and uri rather than mapping strings to assist in resolving the issue.