BIG-IP LTM and ASM - Read xml package to extract information and redirect traffic

If I may add, the XML Content Based Routing feature in LTM can also do this. It's an XML profile and associated iRule. The XPath configuration in the profile can be a little tricky, but once you have it capturing the right value, the iRule is fairly straight forward. here's an example:

when XML_CONTENT_BASED_ROUTING {
    switch $XML_values(0) {
        "12345" {
            pool pool_a
        }
        "67890" {
            pool pool_b
        }
    }
}

As John alludes, depending on when the XML data presents itself in the request, you may have already made a load balancing decision, and/or may need to add some additional persistence tracking if the request is part of a larger session.