Maintenance URL by iRule

If you already have a maintenance.html page on the site, then I would use this

when HTTP_REDIRECT {
    if { not ([HTTP::uri] equals "/maintenance.html") } {
        HTTP::redirect "http://www.abc.com/maintenance.html"
    }
}    

Otherwise, you would have an endless loop because it's not checking if you're already on that page.

If you want a iRule based maintenance page, the Codeshare page has some examples of that.