CCNA 2.2: Interpret Ethernet frame format
A header, data and trailer make up an Ethernet frame.
- 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
- 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
- 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)
- 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.