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What are the recommended MTU settings for ASA C250 iSCSI LUNs with network constraints

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Applies to

  • ASA C250
  • VMware ESXi 8
  • VMFS (Virtual Machine File System) datastores
  • iSCSI protocol

Answer

  • For iSCSI traffic, when using ASA C250 storage arrays, it is recommended to use Jumbo Frames to support larger frame sizes, which typically have an MTU of 9000 bytes, compared to the standard 1500 bytes. 
  • Jumbo Frames can enhance network performance by allowing more data to be sent in a single packet, reducing the number of packets and associated overhead.
  • For example, in the case of network switches with a Layer 2 MTU of 8926 and a Layer 3 MTU of 8900:
    • Given the network constraints with a Layer 3 MTU of 8900, the recommended MTU setting for the ASA C250 when used with iSCSI on ESXi 8 would be to match this constraint, setting the MTU to 8900 bytes.
    • However, due to the additional overhead from TCP/IP and iSCSI headers, it necessary to set the MTU slightly below the maximum to ensure there is enough room for this overhead.
    • A value slightly less than 8900 (e.g., 8850 to 8899) might be more appropriate, but this should be validated through testing in the specific operational environment.
    • Considerations when setting MTU to 8900 include:
      • Packet fragmentation:Ensure all devices along the end-to-end communication path support the 8900-byte MTU to avoid fragmentation and potential performance degradation.
      • Packet drops:Devices that do not support an MTU of 8900 bytes may drop larger frames, increasing the risk of data loss or retransmission overhead.
      • Performance:Larger MTU sizes can improve network throughput but may also increase the likelihood of packet drops during network congestion, leading to increased retransmissions.
      • Compatibility:All network devices must support Jumbo Frames. Devices that do not support the larger MTU will require either a bypass or an adjustment to a smaller MTU size.
      • Configuration consistency:It is crucial to maintain consistent MTU settings across all network devices to avoid communication issues or performance problems.
      • Ensure that the ESXi host's network adapters and any intermediate switches support the configured MTU size.

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