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Is there IP fragmentation by a MTU mismatch between the NAS and NFS client?

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Category:
ontap-9
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nas
Last Updated:

Applies to

  • ONTAP 9
  • NFS
  • MTU (Maximum Transmission Unit)
  • MSS (Maximum Segment Size)
  • IP fragmentation

Answer

  • No, as long as the larger MTU (9000) never sends anything larger then the smaller MTU (1500), there will be no IP fragmentation
    • The NFS client is the only side who initiates the TCP connection (three-way handshake) and the MSS will limit the size of the frames on the NAS side, thus there will be no IP fragmentation

Additional Information

  • The IP fragmentation on the NAS (ONTAP) side can be checked by looking at (look for the keyword "fragments")
    • autosupport: NETSTAT_S.TXT
    • ::> system node run -node * -command "netstat -s -p ip"
  • rfc-editor.org/rfc/rfc9293.txt
    • The maximum size of a segment that a TCP endpoint really sends, the
      ‘effective send MSS’, MUST be the smaller (MUST-16) of the send MSS
      (that reflects the available reassembly buffer size at the remote
      host...) and the largest transmission size permitted by the IP layer
    • SendMSS is the MSS value received from the remote host, or the
      default 536 for IPv4 or 1220 for IPv6, if no MSS Option is
      received
    • If this option is present, then it communicates the maximum receive
      segment size at the TCP endpoint that sends this segment.
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