Maximum Transmission Unit (MTU)

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• standard Ethernet frame has a max length of 1,518 bytes (excluding preamble)
  • each frame has an 18-byte header:
    • 6-byte destination and source MAC address fields
    • 4-byte error checking field
    • 1-byte EtherType field
  • max size of the data payload is 1,500 bytes
    • this limit of the payload is referred to as the maximum transmission unit (MTU)
• when sending a very large data payload, a 1500-byte MTU means using a lot of frames
• a jumbo frame is one that supports a data payload of up to 9,216 bytes
  • reduces the number of frames that need to be transmitted
  • which also reduces the amount of processing that switches and routers need to do
  • reduces the bandwidth requirement by a little:
    • fewer frame headers are being transmitted
  • benefits are disputed, though
    • if implementing jumbo frames, then all hosts and appliances need to support as well
    • also need to support the same MTU
      • can be complex to calculate the MTU if any additional headers are used (e.g., for IPSec)
  • can be configured using the command mtu 9018
    • 9018 is the required size
    • on some appliances, this is configured for the whole system
    • others, can be configured on per-interface basis