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Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

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IP Fragment Handling

The Dell Networking OS supports a configurable option to explicitly deny IP fragmented packets, especially second and subsequent packets.

It extends the existing ACL command syntax with the fragments keyword for all Layer 3 rules applicable to all Layer protocols (permit/deny ip/tcp/udp/icmp).
  • Both standard and extended ACLs support IP fragments.
  • Second and subsequent fragments are allowed because a Layer 4 rule cannot be applied to these fragments. If the packet is to be denied eventually, the first fragment would be denied and hence the packet as a whole cannot be reassembled.
  • Implementing the required rules uses a significant number of CAM entries per TCP/UDP entry.
  • For IP ACL, the system always applies implicit deny. You do not have to configure it.
  • For IP ACL, the system applies implicit permit for second and subsequent fragment prior to the implicit deny.
  • If you configure an explicit deny, the second and subsequent fragments do not hit the implicit permit rule for fragments.

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