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.