Quiz Entry - updated: 2026.07.14
What are the two variants of cut-through switching?
Fast-forward forwards right after reading the destination MAC — lowest latency but may relay errored frames; fragment-free reads and checks the first 64 bytes first, catching collision fragments as a speed/integrity compromise.
| Variant | Bytes Read Before Forwarding | Characteristics |
|---|---|---|
| Fast-Forward | 6 bytes (destination MAC only) | Lowest latency; typical cut-through method; may forward error frames |
| Fragment-Free | 64 bytes | Checks for collision fragments; compromise between speed and integrity |
Fast-Forward:
- Starts forwarding immediately after reading destination MAC
- Errors are detected by the destination NIC, which discards faulty frames
- Best for low-latency requirements
Fragment-Free:
- Reads first 64 bytes before forwarding
- Why 64 bytes? Most network errors and collisions occur in the first 64 bytes
- Catches collision fragments (runt frames) before forwarding
- Better error protection than fast-forward, lower latency than store-and-forward
Go deeper:
Cut-through switching — fragment-free (Wikipedia) — why reading the first 64 bytes lets fragment-free reject collision fragments while staying lower-latency than store-and-forward.