Mellanox ConnectX-5 25GbE Dual-Port SFP28 (MCX512A-ACAT)
Two 25GbE SFP28 ports on a PCIe 3.0 x8 card — the cheapest credible step off 1GbE/10GbE in the used market. The MCX512A-ACAT is the adapter half of the ConnectX-5 family (the VPI parts are the switchable InfiniBand/Ethernet ones), and at current used prices a pair of these costs less than one new 10GbE card. Tested and shipped from Hong Kong.
Fully Tested & Documented · Hong Kong Warehouse Stock · Worldwide Shipping 3–7 Days · 1-Year Warranty
Hardware Gallery
The ConnectX-5 25G, Up Close
ConnectX-5 25G — Dual SFP28, PCIe 3.0 x8
Two SFP28 cages rated at 25GbE each, PCIe 3.0 x8 host interface. The x8 link is sufficient for a single 25G port at line rate; a dual-port card driving both ports flat out is where the x8 host link becomes the limiting factor, so plan your workload one port at a time or confirm your slot is not the constraint.
Bracket, Airflow and Firmware Check
ConnectX-5 cards are designed for server airflow — they run hot enough that a desktop case without directed airflow is not a fair test. We photograph the bracket and heatsink so you can confirm the low-profile or full-height fit you need, and we read the firmware revision off each card before it ships.
2×25G
SFP28 Ethernet ports
PCIe 3.0
x8 host interface
50Gb/s
aggregate port bandwidth
$99
used entry price, Sep 2026
Key Specifications
| Component | Specification |
|---|---|
| Adapter | Mellanox ConnectX-5 25GbE dual-port SFP28 network adapter |
| Part Number | MCX512A-ACAT |
| Ports | 2 × 25GbE SFP28 (SFP28 cages; also accepts SFP+ 10GbE optics) |
| Host Interface | PCIe 3.0 x8 |
| Network Offloads | RDMA over Converged Ethernet (RoCE), SR-IOV, overlay/tunnel offloads |
| Form Factor | Low-profile PCIe adapter with full-height bracket option |
| Cooling | Passive heatsink — requires server chassis airflow |
| Condition | Pre-owned / refurbished tested pulls |
ConnectX-5 25G is the value entry point of the Mellanox 25G line, and its position has changed with price: at $99–$130 used it now undercuts most new 10GbE adapters, so the usual "is 25G worth it?" question mostly answers itself. It pairs naturally with our PowerEdge R750 and ProLiant DL380 Gen10 pages, both of which take the card in a standard PCIe 3.0/4.0 x8 slot. Two things to check before you buy: your switch or DAC/cabling must actually be 25G-capable (SFP28 cages can run 10G optics, so a 10G-only switch is fine but will cap the card), and the card needs real chassis airflow. If you need one 100G port instead of two 25G ports, step up to our ConnectX-5 Ex 100G page.
Transparent Pricing
2026 Reference Pricing by Configuration
Tracked live in the September 2026 collection round — nine asking-price data points, all pre-owned. This is a card where condition spread is small: the whole market sits inside a $31 band, so the buying decision is about optics and bracket, not about finding a bargain.
Pre-Owned ConnectX-5 25G (MCX512A-ACAT)
$99–$130
Nine pre-owned listings tracked, tightest spread of any card in this batch: $99.00, $99.99 ×2, $100.00, $119.00, $119.99 ×2, $124.99, $129.99 — median ≈$119. A band this narrow means there is no distressed end to hunt for; buy on tested condition and bracket fit instead of price.
New / Sealed Retail
On Request
We do not quote a new-price range for this card: the September 2026 collection round returned pre-owned listings only, and we would rather say so than publish a number we did not collect. Ask us and we will price new or sealed stock against live availability.
Asking prices, not transaction prices — volume deals typically close 5–15% below asking. All nine data points are pre-owned listings. Source channel: eBay, sorted by price+shipping, lowest first (first-screen listings). Collected Sep 20, 2026. This entry corresponds to section 37 (Mellanox ConnectX-5 25G, MCX512A-ACAT, dual-port SFP28) of the HKCHL used-server price index.
Origin
Hong Kong Warehouse
Free port — full export documentation
Transit
3–7 Days Air Freight
Tracked, insured, worldwide
Warranty
1-Year Warranty
Diagnostic report in every box
Quality Process
What We Test Before a ConnectX-5 25G Ships
Firmware and Part-Number Read
The on-card part number and firmware revision are read and recorded. Re-marked or mismatched cards are rejected — a card that reports a different PSID than its label is a returns problem waiting to happen.
Link-Up Test at 25G
Each SFP28 port is brought up and verified at 25GbE against a reference switch, port by port. A card that links on port 1 and not port 2 is not shippable.
RoCE / RDMA Bring-Up
RDMA over Converged Ethernet is exercised between two hosts. This is the whole reason to buy a ConnectX-5 rather than a generic 25G NIC, so we verify it rather than assume it.
Thermal Check Under Load
Sustained traffic on both ports with heatsink temperature logged on the passive cooler. These cards are designed for server airflow and we confirm they behave in one.
Bracket and Optic Fit
Bracket type confirmed and the cages checked for damaged latches or bent retaining tabs — a loose cage causes intermittent link drops that look like a driver fault.
Report in the Box
Every unit ships with its per-machine diagnostic report and warranty. Full process: inspection guide.
Frequently Asked Questions
How much is a used Mellanox ConnectX-5 25G in 2026?
Nine pre-owned listings tracked in September 2026 sat between $99 and $130, with a median of about $119. The spread is unusually tight — a $31 band across all nine — which tells you the used market for this specific card has settled. HKCHL quotes are typically at or below the index median, and we will tell you plainly if new stock is the better buy on a given day.
What is the difference between the ConnectX-5 25G and the ConnectX-5 Ex 100G?
Three differences that matter. Bandwidth: the CX-5 gives you two 25GbE SFP28 ports (50Gb/s aggregate) where the CX-5 Ex gives two 100GbE QSFP28 ports (200Gb/s). Host interface: the CX-5 is PCIe 3.0 x8, the CX-5 Ex is PCIe 4.0 x16 — so on a PCIe 3.0-only platform the CX-5 Ex cannot reach its rated throughput no matter what you plug in. Price: used CX-5 runs $99–$130 against $159–$280 for used CX-5 Ex. The honest rule: if your switch and your workload are 25G, the CX-5 Ex is money you do not need to spend. If you are building for 100G storage or GPU fabric, buying CX-5 now means buying twice.
Do I need a 25G switch to use this card?
No, but you should know what you are getting. The SFP28 cages are backward-compatible with 10GbE SFP+ optics, so the card will run at 10G against a 10G switch — you simply do not get 25G. The card also needs DAC or fibre matched to the speed you want. What you cannot do is run 25G over a switch that only does 10G. Tell us your switch model and we will tell you which optics and cables to pair with the card.
Is ConnectX-5 25G still worth buying, or should I go straight to ConnectX-5 Ex?
At $99–$130 used, the CX-5 is now cheaper than most new 10GbE adapters, which makes it the rational pick when your fabric is 25G or when you are upgrading 10G nodes on PCIe 3.0 servers — an R740xd or DL380 Gen10 has no way to feed a 100G card anyway. Go straight to the ConnectX-5 Ex only if you are building for 100G from the start: PCIe 4.0 x16 slots, 100G switching, and either NVMe-over-Fabrics storage or GPU fabric. Buying CX-5 to save money on a 100G plan costs more in the end.
Are refurbished network cards tested before shipping?
Yes, and network cards need it more than most parts. Every card gets a firmware and part-number read, a link-up test at 25G on each port individually, an RDMA/RoCE bring-up test between two hosts, a thermal check under sustained load, and a physical inspection of the SFP28 cages for damaged latches. Cards ship with the test result noted and carry a 1-year warranty.
What optics and cables do I need with the MCX512A-ACAT?
For short in-rack runs, 25G SFP28 DAC (direct-attach copper) is the cheap and reliable answer, and it is what most of our customers order. Beyond roughly 5 metres you want SFP28 optical modules with matched fibre. If you are connecting to a 10G switch instead, use 10G SFP+ DAC or SFP+ optics — the card negotiates down. Tell us the switch, the distance and the port speed you want and we will quote the correct cable with the card.
Get a Current ConnectX-5 25G Quote
Send us the quantity, your switch model and the port speed you are targeting — we confirm MCX512A-ACAT availability, advise on DAC/optic pairing, and can quote the 25G-to-100G step-up side by side. Tested cards ship worldwide from Hong Kong, usually within one business day.
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