Improved Netwatch

This is an improved version of Netwatch that removes false positives. It works by pinging the target IP 5 times. If all 5 time out then the specified action is taken.

        :local i 0; {:do {:set i ($i + 1)} while i < 5) && ([/ping 72.14.207.104 interval=3 count=1]=0)};
       :if ($i=5 && [/ip route get [find comment="Default Route"] disabled]=false) do={:log info "Main Gateway down";
        /ip route set [find comment="Default Route"] disabled=yes}

In this case it searches for a route labelled "Default Route" and disables it if 5 pings in a row to www.google.com time out. To re-enable on 5 succesfull pings you could have some thing like:

       :local i 0; {:do {:set i ($i + 1)} while i < 5) && ([/ping 72.14.207.104 interval=3 count=1]=1)};
      :if ($i=5 && [/ip route get [find comment="Default Route"] disabled]=true) do={:log info "Main Gateway up";
      /ip route set [find comment="Default Route"] disabled=no}


Source: http://wiki.mikrotik.com/wiki/Improved_Netwatch