Hi,
have a look at this URL:
http://www.cisco.com/en/US/products/sw/secursw/ps2120/prod_release_note09186a00800b1138.html
You 'll this info over there:
Preventing Fragmented Packets
By default the PIX Firewall accepts up to 24 fragments to reconstruct a full IP packet. Based on your network security policy, you should consider configuring the PIX Firewall to prevent fragmented packets from traversing the firewall by entering the fragment chain 1 interface command on each interface. Setting the limit to 1 means that all packets must be whole; that is, unfragmented.
For example, to prevent fragmented packets on the outside and inside interfaces enter:
fragment chain 1 outside
fragment chain 1 inside
Continue entering the fragment chain 1 interface command for each additional interface on which you want to prevent fragmented packets.
For more information about the fragment command, refer to the Cisco PIX Firewall Command Reference at: /en/US/products/sw/secursw/ps2120/products_command_reference_chapter09186a00801049b7.html#xtocid15
The PIX Firewall also includes FragGuard for additional IP fragmentation protection. For more information, refer to the Cisco PIX Firewall and VPN Configuration Guide at: http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_62/config/overvw.htm#1046527
Kind Regards,
Tom