After months of stonewalling against allegations by French prop trading HFT Mosaic Finance, Eurex Seems to have caved under pressure. Eurex will shift from Ethernet frame limits to packet limits in its co-location network from April 2026. The change shuts down the ultra-low latency technique Mosaic Finance has labelled Corrupted Speculative Triggering (CST).
Ever since the outbreak of the controversy, Eurex has denied that CST exists.
Eurex has told clients via a circular auutorised by Jonas Ullman, its COO, it will upgrade its co-location 2.0 monitoring infrastructure to Arista’s extensible operating system and a new MetaWatch release. At the same time, it modifies its network access guide and replaces the current Ethernet frame limit with an Ethernet packet limit per order-entry line from 20 April 2026. The hard thresholds, 30,000 messages per second and 600,000 per minute per line, remain unchanged. They will now apply explicitly to all packets detected at the physical layer, including custom Physical Coding Sublayer (PCS) patterns. Sending traffic “in any manner” designed to bypass those limits is therefore explicitly prohibited.
In earlier complaints to the AMF, the Hessen exchange supervisor and ESMA, Mosaic’s president Hugues Morin described CST as a way for some high-frequency trading firms to pre-stream millions of malformed Ethernet packets over multiple order-entry connections. These packets, which did not conform to Ethernet standards, were allegedly invisible to the Arista 7130 MetaWatch counters that enforced Eurex’s frame cap, allowing CST users to exceed the 30,000-frame limit by several orders of magnitude while having to rent several multiple order entry lines to Eurex matching engine.
When a genuine market-data packet finally arrived, CST / AST engines (FPGA or integrated circuits) could reuse the pre-sent preamble and start-of-frame delimiter as the first part of an order message, giving their orders a timestamp a few nanoseconds before the market-data event they were reacting to. At the speed of light, these racing orders start 1 to 2 meters in front of regular speculatively triggered orders. Mosaic’s lab tests seen by Global Trading suggested a gain of up to 6.4 nanoseconds and a fourfold increase in “hit ratio”, the probability of being first into the matching engine. Mosaic has said this advantage could have cost market participants up to €100 million of P&L.
Mosaic argued that CST breached both the quantitative limits and the qualitative requirements of Eurex’s network access guide. It required participants to “use network protocols in an orderly fashion” and not to send corrupted Ethernet frames such as shortened or padded packets. Mosaïc supplied regulators with a MetaWatch configuration it said would allow Eurex to count corrupted packets and an expert opinion from the University of New Hampshire Inter operability Laboratory confirming that CST packets were not Ethernet-compliant.
Eurex, by contrast, had maintained in correspondence and court proceedings that the practice, which it described as Augmented Speculative Triggering (AST), was compliant as long as its existing monitoring did not trigger an alarm, and that CST packets did not reach the trading system but were discarded by switches. A case brought by Mosaic before the administrative court in Wiesbaden was rejected on procedural grounds, without a ruling on the substance of the allegations.
Read more: Eurex denies getting swamped by “corrupted” messages as HFT dispute escalates
Lawyers and ultra-low frequency trading specialist firms from Jump Trading to specialist vendors from Exegy to McKay brothers are now likely to want to know what this means for the past. While the Network Access Guide has long banned corrupted Ethernet frames in principle, the move from frame-based to packet-based monitoring seems like an implicit admission that the old setup could not reliably see or count them and so could not prevent CST in practice.
A Eurex Spokesman told Global trading: “The circular does not introduce a prohibition to Advanced Speculative Triggering (AST – not CST). It reflects our regular, continuous improvement of our monitoring infrastructure and clarifies how existing technology‑related limits are enforced. Specifically, Eurex is extending the current Ethernet Frame Limit to an Ethernet Packet Limit, so that more network patterns are explicitly in scope of the same per‑second and per‑minute thresholds. “
Mosaïc Finance was not immediately available for comments at the time of writing
