iRule to redirect if value does not exist in cookie

A user can have multiple cookies and each of those cookies can have separate values, so to say "looking for a value that should be present in a cookie", would indicate that you're looking for a cookie's value, without specifying which cookie to inspect.

I believe what that Netscaler config is doing is checking for the existence of the SMSESSION cookie itself in the request (not necessarily the value in that cookie), which in this case would be similar to the first example above.

when HTTP_REQUEST {    
    if { not ( [HTTP::cookie exists SMSESSION] ) } {        
        HTTP::redirect "https://differenturlloginpage.com"            
    }        
}