Calculate the FCS number from a frame ethernet

Question

I've the following frame ethernet:
08 00 20 0A 70 66 08 00 20 0A AC 96 08 00 45 00 00 28 A6 F5 00 00 1A 06 75 94 C0 5D 02 01 84 E3 3D 05 00 15 0F 87 9C CB 7E 01 27 E3 EA 01 50 12 10 00 DF 3D 00 00 20 20 20 20 20 20 9B 52 46 43
Where 0x9B524643 is the FCS number. Using this tool (https://www.gasmi.net/hpd/) I can see that this FCS doesn't correspond to the real FCS that would be 0x5A05DEFA. How is the real FCS calculated (0x5A05DEFA)? Thanks

Answer 1

The FCS is a CRC over all fields (except the FCS) with the polynomial

G(x) = x³² + x²⁶ + x²³ + x²² + x¹⁶ + x¹² + x¹¹ + x¹⁰ + x⁸ + x⁷ + x⁵ + x⁴ + x² + x + 1

with the procedure detailed in IEEE 802.3 Clause 3.2.9 - complement first 32 bits, run the polynomial, complement the result and transmit most significant to least significant bit - this is reverse the normal Ethernet order.