How to clear Don't Fragment (DF) bit

When client ping 1500 Byte (some is 1482) to that problem virtual server (letter X.X.X.X) and X.X.X.X reply with DF bit

but first hop is switch cisco nexus 7000 and (probaly checkpoint firewall) and what it does is checkpoint drop packet and sent " ICMP Destination unreachable (Fragmentation needed) " to X.X.X.X

this picture is some of all topology

http://upic.me/i/dp/f5-firsthop.png

ps. In linux based ICMP request will set DF bit , but Did ICMP reply from F5 is DF bit set ? >>> this is problem make checkpoint drop ICMP reply packet (cause ICMP reply have more than 1500 bytes and DF bit is set (so it'll not fragmented)) ......but i didn't sure Is F5 resend smaller ICMP reply. I'll investigate more.