What are the ethernet CRCs and how they work?
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