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A forwarding engine runs out of packets per second, not bits. Drag the frame size and watch what the same engine is worth in Gbit/s — and what a datasheet number quietly assumes.
at 64-byte frames, with the engine held at 50 Mpps
The packet rate is what the engine is held to. The bit rate is arithmetic on top of it: wire_bits = (frame_bytes + 20) × 8, then Gbps = pps × wire_bits ÷ 1e9. The 20 bytes are preamble, start-of-frame delimiter and inter-frame gap. Past about 980-byte frames the adapters saturate before the engine does.
Every Ethernet frame shares the wire with a 7-byte preamble, a start-of-frame delimiter and a 12-byte inter-frame gap, so line-rate arithmetic has to count them:
wire_bits = (frame_bytes + 20) × 8
Gbit/s = pps × wire_bits ÷ 1,000,000,000
A data plane does roughly the same work for a 64-byte frame as for a 1,518-byte one: a lookup, a rewrite, a queue decision. The honest capacity number is therefore a packet rate, and line rate follows the frame size rather than the other way round.
The smallest Ethernet frame puts the most packets on the wire per bit, so it exposes an engine that cannot keep up. A vendor quoting only large-frame throughput is quoting the easy case.
A traffic mix closer to what a residential edge really carries, averaging around 340 bytes. It sits between the worst case and the marketing case, and it is the one worth planning against.
A throughput figure with no frame size and no list of the features that were enabled is not a measurement. Ask for packets per second with subscriber termination, NAT, shaping and filtering all switched on.
BNGSOFT publishes one packet rate and derives everything else from it: 50 Mpps per box with PPPoE, CGNAT, hierarchical QoS and ACLs all enabled. Through the formula above that is 33.6 Gbit/s at 64-byte frames, roughly 110 Gbit/s at 256 bytes and about 144 Gbit/s at IMIX. The 400 Gbit/s on the spec sheet is port capacity from two dual-port E810 cards, not forwarding throughput — different claims, kept apart. How the XDP data plane reaches it.
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