61-2
Cisco ASA 5500 Series Configuration Guide using ASDM
Chapter61 Using Protection Tools
Configuring the Fragment Size
•Anti-Spoofing Enabled—Shows whether an interface has Unicast RPF enabled, Yes or No.
•Enable—Enables Unicast RPF for the selected interface.
•Disable—Disables Unicast RPF for the selected interface.
Configuring the Fragment Size
By default, the ASA allows up to 24 fragments per IP packet, and up to 200 fragments awaiting
reassembly. You might need to let fragments on your network if you have an application that routinely
fragments packets, such as NFS over UDP. However, if you do not have an application that fragments
traffic, we recommend that you do not allow fragments through the ASA. Fragmented packets are often
used as DoS attacks.
To modify the IP fragment database parameters of an interface, perform the following steps:
Step1 Choose the Configuration > Firewall > Advanced > Fragment pane, choose the interface to change in
the Fragment table, and click Edit.
The Edit Fragment dialog box appears.
Step2 In the Size field, set the maximum number of packets that can be in the IP reassembly database waiting
for reassembly. The default is 200.
Step3 In the Chain field, set the maximum number of packets into which a full IP packet can be fragmented.
The default is 24 packets.
Step4 In the Timeout field, set the maximum number of seconds to wait for an entire fragmented packet to
arrive.
The timer starts after the first fragment of a packet arrives. If all fragments of the packet do not arrive
by the number of seconds specified, all fragments of the packet that were already received will be
discarded. The default is 5 seconds.
Step5 Click OK.
Step6 Click Apply.
Step7 To view the fragment statistics, click Show Fragment. See the “Show Fragment” section on page61-2
for more information.

Show Fragment

The Configuration> Properties > Fragment > Show Fragment pane displays the current IP fragment
database statistics for each interface.
Fields
•Size—Display only. Displays the number of packets in the IP reassembly database waiting for
reassembly. The default is 200.
•Chain—Display only. Displays the number of packets into which a full IP packet can be fragmented.
The default is 24 packets.