Configuring IP Router Parameters
54 7705 SAR OS Router Configuration Guide
• For destination NAT (port forwarding) packets traversing from a public network to
a private network and destined to a local IP address on the 7705 SAR, fragmented
packets that do not contain the UDP header are extracted to the CSM for processing
and an ICMP error message is sent to the sender.
On the 7705 SAR-8 and 7705 SAR-18, in addition to the condition requiring the fragment
containing the header to arrive first, all fragments of a given packet must arrive on the same
adapter card for processing.
For example, if an upstream router in the network performs IP ECMP load balancing and the
load balancing takes Layer 4 port information into account in its hashing algorithm, it is
possible that the algorithm will put the packet fragment that contains the UDP header
information (that is, the first fragment of the packet) on one Layer 3 interface and the packet
fragment that does not contain the UDP information (that is, the subsequent fragments of the
packet) on a different Layer 3 interface. When packet fragments traverse different Layer 3
interfaces, depending on the interface latency, it is possible that the fragments will arrive out
of sequence at the destination. Should this occur and the non-UDP packet fragments arrive
ahead of the packet fragment that contains the UDP header, the 7705 SAR router will discard
the packets that do not have UDP information because they arrived out of sequence. In order
to avoid fragment sequencing issues caused by Layer 4 load balancing hashing algorithms
such as IP ECMP or LAG, it is recommended that these algorithms be disabled on all routers
in the network if it is foreseen that packet fragments might be arriving on the 7705 SAR
router.
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