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Cisco Bug: CSCup77939 - Fragment packets not containing L4 header are dropped

Last Modified

May 22, 2017

Products (84)

  • Cisco IOS
  • Cisco VG204XM Analog Voice Gateway
  • Cisco 888W Integrated Services Router
  • Cisco 886VA-CUBE Integrated Services Router
  • Cisco 1905 Serial Integrated Services Router
  • Cisco 2951 Integrated Services Router
  • Cisco 812 CiFi Integrated Services Router
  • Cisco 886VAG 3G Integrated Services Router
  • Cisco 819 Hardened Integrated Services Router
  • Cisco C897VA Integrated Services Router
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Known Affected Releases


Description (partial)

Fragment packets, which do not contain L4 header, are dropped on a PAT router.

In the customer site, large SNMP trap data is regularly generated on a server and
the trap is sent in 3 fragmented packets in the direction of outside to inside.
The size of packets are 1146, 1194 and 304 bytes respectively.

The first packet contains UDP header and the NAT router has no problem forwarding it.
The second and the third packets does not contain UDP header and the NAT router discard them.

The problem is seen when a inside source static PAT rule is configured on a NAT router
and fragmented packets matching the PAT rule are transmitted in the direction of NAT outside to NAT inside.

Under the condition, the NAT router cannot forward fragmented packets which does not contain L4 header
while it has no problem forwarding the fragmented packet with L4 header.

     L3     L3   L3/L4
   -----> -----> ----->
   ----------(outside) NAT Router (inside) ----------
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Bug Details Include

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