Size the 4-byte Ethernet FCS, count the bits it protects and estimate how many corrupt frames slip past CRC-32.
A frame's error probability is its length in bits multiplied by the link bit error rate. CRC-32 catches every burst up to 32 bits and every odd number of bit errors, but a residual 1-in-4.3-billion fraction of corrupted frames still passes, which is what the undetected figure counts. The residual error rate is the number auditors ask about when Ethernet carries storage or financial traffic, because a frame that passes FCS but is corrupt becomes silent data corruption further up the stack.
Frame Check Sequence
The FCS is a 32-bit CRC over the frame from the destination MAC to the end of the payload, so a corrupted frame escapes detection with probability 2^-32.
Undetected error rate
P(frame in error) = frame bits × BER; undetected frames = errored frames × 2^-32.
The FCS is a 32-bit CRC over the frame from the destination MAC to the end of the payload, so a corrupted frame escapes detection with probability 2^-32. A frame's error probability is its length in bits multiplied by the link bit error rate. CRC-32 catches every burst up to 32 bits and every odd number of bit errors, but a residual 1-in-4.3-billion fraction of corrupted frames still passes, which is what the undetected figure counts.
The residual error rate is the number auditors ask about when Ethernet carries storage or financial traffic, because a frame that passes FCS but is corrupt becomes silent data corruption further up the stack.
This calculator takes 3 inputs: Payload size (L3 packet), Link bit error rate (10^x), Frame rate on the link. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Dropped. Ethernet's FCS is detection-only, with no forward error correction, so the receiving NIC counts a CRC error and discards the frame — recovery is left to TCP or the application.