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What are the ethernet CRCs and how they work?

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Applies to

  • Ethernet protocol. IEEE 802.3
  • CRC or Frame check sequence

Answer

This is the structure of an ethernet frame: 

Layer 

Preamble 

Start frame delimiter 

MAC destination 

MAC source 

802.1Q tag (optional) 

Ethertype (Ethernet II) or length (IEEE 802.3) 

Payload 

Frame check sequence (32‑bit CRC) 

Interpacket gap 

 

7 octets 

1 octet 

6 octets 

6 octets 

(4 octets) 

2 octets 

46-1500 octets 

4 octets 

12 octets 

Layer 2 Ethernet frame 

 

← 64–1522 octets → 

 

Layer 1 Ethernet packet & IPG (PHY) 

← 72–1530 octets → 

← 12 octets → 

Additional Information

From the sending side: 

  • Ethernet frame is ready 
  • CRC is calculated and appended 
  • Ethernet frame is sent to PHY 

And, from the receiving side: 

  • Frame is received from PHY 
  • NIC FW checks CRC against payload 
  • Good CRC: frame is handed over to driver 
  • Bad CRC: frame is dropped 

 

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