Both are retired enterprise SFP+ cards with drivers in the mainline Linux kernel - mlx4 for the ConnectX-3, ixgbe for the X520 - so on a Linux-based system with a direct-attach cable, buy whichever is cheaper. Today the ConnectX-3 starts 37% below the X520. Choose ConnectX-3 if you are using third-party optical modules, because Intel-branded X520s enforce a transceiver whitelist. Choose the X520 for a machine you never want to revisit, because ConnectX-3 is end-of-life at NVIDIA.
Prices updated · live figures update every 4-5 hours
mlx4 in-kernel · end-of-life at NVIDIA · relaxed about third-party modules
ixgbe in-kernel · no EOL notice · Intel-branded cards enforce a transceiver whitelist
| Card | Ports | Interface | Condition | Price | |
|---|---|---|---|---|---|
| Intel X520 10Gtekseller claims this chipset | 2 | SFP+ | Refurb | $44.41 | Buy |
| Intel X520 10Gtekseller claims this chipset | 1 | SFP+ | New | $44.99 | Buy |
| Intel X520 Intel | 2 | SFP+ | Refurb | $45.00 | Buy |
| Intel X520 Intel | 2 | SFP+ | Refurb | $45.00 | Buy |
| Intel X520 Intel | 2 | SFP+ | Refurb | $49.99 | Buy |
| Intel X520 10Gtekseller claims this chipset | 2 | SFP+ | New | $51.99 | Buy |
| Intel X520 Intel | 2 | SFP+ | New | $58.99 | Buy |
| Intel X520 Intel | 2 | SFP+ | New | $63.99 | Buy |
| Intel X520 Intel | 2 | SFP+ | New | $64.99 | Buy |
Worth noting on the Intel side: the cheapest Intel X520 is $45.00, while the cheapest third-party board asserting the same 82599 silicon is $44.41chipset attribution is the seller's. Those boards often do not implement the transceiver whitelist, which makes them a different proposition from the first-party card rather than simply a cheaper one.
This is the single most-repeated question in home 10GbE discussion, and it usually gets answered with brand loyalty. The documented facts are more useful and they point in different directions depending on what you are building, which is why the answer below is by platform rather than a single winner.
Both cards are old enterprise SFP+ adapters that entered the retail market in volume because datacentres retired them. Both are cheap. Both have drivers in the mainline Linux kernel. Neither will disappoint you on capability, because at 10GbE the card is almost never the constraint.
The differences that matter are three: which one your operating system supports without argument, which one will accept the module you want to plug into it, and how much longer each is likely to be supported.
The ConnectX-3 uses the mlx4 driver. The Intel X520, built on the 82599 controller, uses ixgbe. Both ship in the mainline Linux kernel, which is the fact that actually determines whether a Linux-based system recognises the card - and it means TrueNAS, Proxmox, Unraid and ordinary Linux distributions see either card without you installing anything.
The distinction worth knowing is what else those drivers carry. ixgbe also drives the X540 and X550, so it is exercised by a wide range of current hardware and has an obvious reason to keep being maintained. mlx4 serves the ConnectX-3 generation specifically; newer NVIDIA adapters moved to mlx5, which means mlx4's user base only shrinks from here.
Neither driver is going anywhere imminently. Drivers leave the mainline kernel slowly and with a deprecation cycle, not overnight. But if you are asking which is more likely to still be present in a decade, ixgbe has the stronger position, and that is an inference from what else it drives rather than an announcement anyone has made.
On Windows the picture is less comfortable for both, and it varies by exact part number and vendor branding. If Windows is the target, check the maker's current driver page for your specific model before buying - in-kernel Linux support tells you nothing about it.
ConnectX-3 is end-of-life at NVIDIA. The X520 does not carry an equivalent notice. That is a real difference and it is the strongest single argument for the Intel card.
It is also routinely overstated. End-of-life means the maker has stopped developing firmware and drivers for the product. It does not mean the card stops working, it does not mean it has been removed from operating systems, and it does not mean anything will change on a particular date. A card whose driver is in the mainline kernel keeps working for as long as that driver is there.
The honest way to weigh it is by time horizon. For a machine you expect to rebuild or replace within a few years, the EOL notice should not change your decision and the cheaper card wins. For something you intend to install and never revisit, the card without the notice is buying you a longer runway, and the price difference is small next to the cost of revisiting the decision.
This is where the two cards genuinely diverge in day-to-day use, and it is the difference most likely to cost you an afternoon.
Intel-branded X520 cards have historically enforced a whitelist of approved transceivers in firmware, refusing modules that do not identify as Intel parts. This is documented behaviour and a deliberate policy rather than a defect. Mellanox cards have a reputation for being considerably more relaxed about what they will accept, and that reputation is a large part of why ConnectX-3 became the community default for home builds despite the EOL notice.
Two things soften this. A passive direct-attach cable is far less likely to run into the problem than a separate optical module, and a direct-attach cable is what most two-machine builds should use anyway. And third-party boards built on the 82599 controller frequently do not implement the whitelist at all - which is one reason those boards are popular beyond simply being cheaper. Whether a given board carries the silicon its listing names is the seller's claim, and we badge it.
If you already own modules, this is the deciding factor. If you are buying a direct-attach cable, it is close to irrelevant.
For a Linux, TrueNAS, Proxmox or Unraid box with a direct-attach cable: either card, and the cheaper one on the day. Both drivers are in-kernel, the whitelist question does not arise with a DAC, and the EOL notice does not affect a machine you will rebuild eventually. This is the majority case and the honest answer is that the choice barely matters.
For a build using third-party optical modules: ConnectX-3, or a third-party 82599 board rather than an Intel-branded X520. The whitelist is the whole reason.
For a machine you want to leave untouched for as long as possible: X520, for the absence of an EOL notice and because ixgbe also drives current hardware and therefore has an obvious future.
For Windows: check the specific part number against the maker's current driver page before committing to either. This is the case where assuming costs you the most.
The ConnectX-3 page and the X520 page cover each card in full · the two-machine build uses whichever you pick · transceivers is where the whitelist question actually bites.
For a Linux-based system using a direct-attach cable, whichever is cheaper on the day: both drivers are in the mainline kernel and the differences do not surface. Choose ConnectX-3 if you are using third-party optical modules, because Intel-branded X520 cards enforce a transceiver whitelist. Choose the X520 if you want the machine untouched for a decade, because it carries no end-of-life notice.
Less than it sounds. EOL means NVIDIA has stopped developing firmware and drivers for it - not that the card stopped working or that operating systems dropped it. The mlx4 driver is in the mainline Linux kernel and in-kernel drivers are removed slowly and with notice. It is a question about your time horizon rather than about whether the card works today.
ConnectX-3 uses mlx4, the X520 uses ixgbe. Both ship in the mainline Linux kernel. ixgbe also drives the X540 and X550, so it is exercised by current hardware; mlx4 serves the ConnectX-3 generation only, since newer NVIDIA adapters moved to mlx5.
Intel-branded cards have historically applied a firmware whitelist and reject modules that do not identify as approved Intel parts. It is documented policy, not a fault. A passive direct-attach cable largely avoids the issue, and third-party boards built on the same 82599 controller often do not implement the whitelist - though whether a given board carries that silicon is the seller's claim.
Both are 10GbE SFP+ adapters, so the link speed is identical. At 10GbE the card is almost never the constraint - the storage at either end usually is. Choosing between them on performance grounds is choosing on the wrong axis.
It moves, which is why the tables above are live rather than a number written into this page. Historically the ConnectX-3 has been the cheaper of the two, and the very cheapest listings in this category are typically ConnectX-3 variants. Check the current figures above.
Specification claims on this page were checked against the pages below on 4 August 2026. Where a widely-repeated figure did not survive that check, we report what the source says now and say so in the text rather than repeating the familiar number.