Introduction
Mellanox ConnectX network adapters are the gold standard for high-performance datacenter networking. From 25GbE workloads to 200Gb/s InfiniBand clusters, the ConnectX family covers every tier of enterprise networking. But with so many models available, choosing the right one can be confusing.
This guide breaks down the ConnectX-4, ConnectX-5, and ConnectX-6 families by speed, use case, and key specifications — so you can make the right call for your infrastructure.
ConnectX-4 Lx — 25GbE Entry Point
The ConnectX-4 Lx is the workhorse of 25GbE datacenter networking. It supports dual-port 25GbE over SFP28 and is widely deployed in hyperscale and enterprise environments.
- Key models: MCX4121A-ACAT (25GbE dual-port), MCX4131A-GCAT (40GbE single-port)
- Interface: PCIe 3.0 x8
- Features: RDMA over Converged Ethernet (RoCE v2), SR-IOV, DPDK
- Best for: VM host networking, storage replication, NVMe-oF initiators
ConnectX-5 — 100GbE Performance
ConnectX-5 delivers 100GbE performance with full RoCE v2 and NVMe-oF support. It is the most widely deployed 100GbE NIC in enterprise datacenters today.
- Key models: MCX516A-CCAT (100GbE dual-port QSFP28), MCX512A-ACUT (25GbE dual-port SFP28)
- Interface: PCIe 3.0 x16 (dual 100G) or PCIe 3.0 x8 (dual 25G)
- Features: Hardware offloads, VXLAN/NVGRE, GPUDirect RDMA
- Best for: AI/ML training clusters, NVMe-oF storage fabrics, high-frequency trading
ConnectX-6 — 200Gb/s HDR InfiniBand and 100GbE
ConnectX-6 is NVIDIA’s flagship adapter, supporting both 200Gb/s HDR InfiniBand and 100GbE Ethernet in a single card. It is the adapter of choice for GPU clusters and HPC environments.
- Key models: MCX653105A-ECAT (100GbE/HDR100 single-port), MCX653105A-HDAT (200Gb/s HDR single-port), MCX653106A-HDAT (200Gb/s HDR dual-port)
- Interface: PCIe 4.0 x16
- Features: In-network computing, SHARP aggregation, GPUDirect RDMA, RoCE v2
- Best for: NVIDIA DGX clusters, HPC, AI training, InfiniBand storage fabrics
Choosing the Right ConnectX Model
- VM host networking / general server: ConnectX-4 Lx 25GbE (MCX4121A-ACAT)
- NVMe-oF storage fabric: ConnectX-5 100GbE (MCX516A-CCAT)
- AI/GPU cluster: ConnectX-6 200Gb/s HDR (MCX653105A-HDAT)
- Mixed Ethernet + InfiniBand: ConnectX-6 (supports both protocols)
PCIe Generation Compatibility
ConnectX-4 Lx and ConnectX-5 use PCIe 3.0, while ConnectX-6 requires PCIe 4.0 for full bandwidth. ConnectX-6 cards are backward compatible with PCIe 3.0 slots but will operate at reduced throughput. Always verify your server’s PCIe slot generation before ordering.
Buy Used Mellanox ConnectX Adapters
Zhongxin Data stocks a full range of used Mellanox ConnectX-4, ConnectX-5, and ConnectX-6 adapters. All cards are original original-vendor hardware, fully tested, and firmware-verified. Worldwide shipping from Hong Kong, China with warranty included.
Contact us for bulk pricing or compatibility assistance.
Mapping the ConnectX families to speed classes
- ConnectX-4 and ConnectX-5 cover 25GbE and 100GbE, with ConnectX-5 adding 200GbE variants
- ConnectX-6 covers 100GbE and 200GbE with a newer PCIe generation and better offload
- Match the family to the link rate you actually terminate, not to the newest part number
Choosing ports, lanes and power budget
- Dual-port cards use more PCIe lanes and two switch ports — count both against the chassis budget
- A 25GbE adapter often runs on fewer lanes, which matters when the server is dense with GPUs
- Check the auxiliary power connector requirement before you plan the cable run
Cabling and the server-compatibility checklist
- QSFP28 for 100GbE and QSFP56 for 200GbE, with breakout options where the switch supports them
- Check the bracket type and the cage height against your chassis and your riser layout
- On a used card, the firmware level decides which drivers and features are available to you
Related reading
- MCX4121A 25GbE adapter — the 25GbE class
- MCX516A-CCAT 100GbE adapter — the mainstream 100GbE class
- ConnectX adapter comparison — the model-by-model comparison
- Network adapters — category hub
Frequently Asked Questions
Which Mellanox ConnectX NIC should I buy?
Match the adapter to three things: the link rate you need (25G, 100G or 200G), the PCIe generation your server provides, and the number of switch ports you want to consume. For 100GbE on PCIe 3.0 x16 the MCX516A-CCAT is the common answer; for 200GbE on PCIe 4.0 x16 you need a ConnectX-6 card. Tell us the server model and the switch model and we will confirm the combination.
Is a ConnectX-5 still a good choice in 2026?
Yes, for 100GbE it remains a well-supported and widely stocked part, and the ConnectX-6 does not make it obsolete. Choose ConnectX-5 when your server has PCIe 3.0 slots or your fabric is 100GbE. Reach for ConnectX-6 when you need 200GbE links, PCIe 4.0 bandwidth, or a newer firmware baseline.
What does the CCAT versus CDAT suffix mean?
It identifies the card variant: the physical medium, the speed grade and the PCIe configuration. A CCAT card is typically the copper-cable-oriented variant, while CDAT variants carry a different bracket and connector arrangement for fibre media. The suffix tells you the physical fit, so match it to your cabling and chassis rather than relying on the product name alone.
Can I run a 25GbE adapter on a 100GbE switch?
Yes, provided the switch supports the breakout. A 100GbE switch with QSFP28 cages can break out to 25GbE on compatible ports, and the card negotiates down. This is the standard way to stage a migration, and it means you do not need 25GbE switches to start.
How many PCIe lanes does a dual-port ConnectX card use?
Depending on the card, a dual-port adapter can consume x16 and leave the second slot to another device, or it can require a second slot for the additional lanes. This matters in a dense server with limited slots. Tell us the server model and we will confirm the lane and slot requirement for the exact part number.
How do you test used Mellanox NICs?
We clean, re-flash and bench-test on our own test line, then confirm the part number, port count, negotiated link rate and firmware level, and we read the error counters so a card that links but errors does not get listed. Ask for photos of the exact unit and its firmware level before you commit.