CCNA 2.2: Interpret Ethernet frame format

Overview:

A header, data and trailer make up an Ethernet frame.

Study Notes:

  • A data unit on an Ethernet link transports an Ethernet frame as its payload.
  • An Ethernet frame is preceded by a preamble and start frame delimiter (SFD), which are both part of the Ethernet packet at the physical layer.
  • In the below image, focus on the IEEE 802.3 (Revised 1997) depiction of an Ethernet Frame


Ethernet Frame Diagram

 

Preamble

  • 7 bytes
  • Alternating 1s and 0s to make it easy for devices to synchronize their clocks

 

Start Frame Delimiter (SFD)

  • 1 byte
  • Provides byte level synchronization and marks a new incoming frame

 

Destination MAC address

  • 6 bytes
  • Identifies the intended recipient of this frame

 

Source MAC address

  • 6 bytes
  • Identifies the sender of this frame

 

Length

  • 2 bytes
  • Values of 1500 and below mean that it is used to indicate the size of the payload in octets
  • Defines the length of the data field of the frame (either length or type is present, but not both)

 

Type

  • 2 bytes
  • Values of 1536 and above indicate that it is used as an EtherType, to indicate which protocol is encapsulated in the payload of the frame.
  • Defines the type of protocol listed inside the frame (either length or type is present, but not both)
  • Tag (optionally applied)
  • 4 bytes with the first 2 bytes doubling as the Type field
  • IEEE 802.1Q or IEEE 802.1ad tags are applied to indicate VLAN membership
  • IEEE802.1p tag is applied to indicate priority

 

Data and Padding

  • 42 bytes minimum when an 802.1Q tag is applied
  • 46 bytes minimum without an 802.1Q tag applied
  • 1500 bytes maximum
  • Padding bytes are added if actual payload is not equal to 1500 bytes
  • Jumbo frames can be implemented to increase the maximum payload size
  • Payload is all the data from higher layers, fragmented to fit the max payload size

 

Frame Check Sequence (FCS)

  • 4 bytes
  • A cyclic redundancy check (CRC) that allows detection of corrupted data within the entire frame as received on the receiver side
  • The FCS value is computed as a function of the protected MAC frame fields: source and destination address, length/type field, MAC client data and padding (that is, all fields except the FCS)
  • Running the CRC algorithm over the received frame data including the CRC code will always result in a zero value for error-free received data, because the CRC is a remainder of the data divided by the polynomial.
2 comments
  1. Alexandru Cehan
    Alexandru Cehan
    August 29, 2019 at 8:49 am

    “Padding bytes are added if actual payload is not equal to 1500 bytes”, this doesn’t make sense, so the minimum size is actually 1500 bytes?

    • Joe Barger (CCNP/CCDP)
      Joe Barger (CCNP/CCDP) • Post Author •
      August 31, 2019 at 10:40 am

      Data bytes + Padding bytes = 1500.
      If there are 1500 data bytes in that particular frame then 0 padding bytes will be added. If there are 1400 data bytes then 100 bytes of padding will be added, etc.

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