iRule Redirect loop

Not totally sure of your requirement, but I will still a venture a reply, if only serving as a pointer:

when HTTP_REQUEST {
    if { [HTTP::host] equals "test.tst.train123.com" } {
        switch -exact [string tolower [HTTP::path]] {
            "/" {
                     HTTP::redirect "https://test.tst.train123.com/Trace123"
            }
            default {
                        pool pool_trace123
            }
        }
    }
}

An LTM policy would actually be a better option, and its building process would guide you through it in a more transparent way and therefore be helpful to you.

[Edited]