QDD-4ZQ100-CU1M=: Cisco’s High-Density QSFP-DD Passive Copper Cable for 400G Data Center Interconnects

​​Technical Architecture and Design Philosophy​​

The ​​QDD-4ZQ100-CU1M=​​ is a Cisco-certified ​​QSFP-DD (Quad Small Form-Factor Pluggable Double Density)​​ passive copper cable engineered for high-speed, short-reach interconnects in hyperscale data centers. As part of Cisco’s 400G ecosystem, this 1-meter cable supports ​​4x100G NRZ​​ or ​​1x400G PAM4​​ signaling, enabling cost-effective spine-leaf topology scaling. Key specifications include:

• ​​Data Rate​​: 400 Gbps aggregate (4x100G lanes) with 25.78125 Gbps per channel
• ​​Cable Type​​: Passive Twinaxial (PAM4 for 400G, NRZ for 100G breakout)
• ​​Connector​​: QSFP-DD MSA-compliant, 76-pin (backward compatible with QSFP28/QSFP56 ports)
• ​​Power Consumption​​: 1.5W max (passive design)
• ​​Compliance​​: IEEE 802.3bs, CMIS 4.0, and RoHS v3

Cisco’s design brief emphasizes its role in ​​Nexus 9000 Series​​ deployments, particularly the N9K-X9716D-GX line card, where port density and thermal efficiency are critical.

​​Target Applications and Deployment Scenarios​​

The QDD-4ZQ100-CU1M= addresses three critical challenges in modern data centers:

​​1. Hyperscale Spine-Leaf Clos Architectures​​
In Cisco Nexus 9336C-FX2 switches, the cable enables ​​non-blocking 400G spine connections​​ with <100ns latency, ideal for high-frequency trading (HFT) and AI/ML workloads.

​​2. Storage Area Network (SAN) Backbones​​
When used with Cisco MDS 9700 Directors, the cable supports 128 GFC Fibre Channel over Ethernet (FCoE), reducing ISL (Inter-Switch Link) costs by 40% compared to optical transceivers.

​​3. GPU Cluster Interconnects​​
Provides low-jitter (<0.5 UI) connectivity between NVIDIA DGX A100 systems and Cisco UCS X-Series servers, crucial for distributed training jobs.

​​Compatibility and Hardware Validation​​

Validated for use with:

• ​​Switches​​: Nexus 92300YC, 9364C, 9336C-FX2 (with NX-OS 9.3(5)+)
• ​​Routers​​: ASR 9904 with 400G CFP2-DCO modules
• ​​Servers​​: UCS C480 M5 ML in EIA-310-D racks

A critical limitation: The cable does ​​not support Auto-Negotiation​​—speed and protocol must be manually configured via Cisco NX-OS CLI.

​​Key Differentiators from Active Optical Cables (AOCs)​​

​​1. Cost Efficiency​​
At 60% lower cost per port than 400G-SR4 optics, the passive copper design eliminates DSP/SerDes components, making it ideal for <3m interconnects.

​​2. Thermal Resilience​​
Operates at 0°C to 70°C without airflow—crucial for rear-panel connections in OCP (Open Compute Project) racks.

​​3. Error Rate Performance​​
Achieves ​​BER <1e-15​​ with Cisco’s Forward Error Correction (FEC) in PAM4 mode, outperforming third-party DACs limited to 1e-12.

​​Installation and Signal Integrity Best Practices​​

• ​​Bend Radius​​: Maintain >30mm to prevent impedance mismatches
• ​​Cable Management​​: Use horizontal managers to avoid crosstalk in 1U spaces
• ​​Grounding​​: Ensure rack-side grounding via Cisco’s AOC-GND-KIT to mitigate EMI in 400G PAM4 deployments

​​Purchasing and Warranty Considerations​​

The QDD-4ZQ100-CU1M= includes a ​​lifetime limited warranty​​, covering connector wear and shield integrity. Key procurement factors:

• ​​Minimum Order Quantity (MOQ)​​: 10 units for volume pricing
• ​​Lead Time​​: 2–4 weeks due to cobalt shortage in twinax shielding

For verified stock and Cisco Smart Net Total Care registration, this link connects to authorized resellers.

​​Addressing Critical User Concerns​​

​​Q: Can it support 200G speeds via breakout configurations?​​
A: Yes. Split into 2x200G (QSFP56 to QSFP56) or 4x100G (QSFP-DD to QSFP28) using Cisco’s ​​Breakout Compatibility Matrix​​.

​​Q: How does it handle signal attenuation over 1m?​​
A: Built-in ​​28 AWG conductors​​ limit insertion loss to 0.5 dB/m at 26 GHz, negating the need for signal boosters.

​​Q: Is it compatible with non-Cisco switches like Arista 7060X2?​​
A: Limited to link training mode 1 (LT1). For full CMIS 4.0 support, use Cisco Nexus/Catalyst switches.

​​Future-Proofing in 800G-Ready Infrastructures​​

While designed for 400G, the QDD-4ZQ100-CU1M= lays groundwork for:

• ​​800G Upgrades​​: Compatible with Cisco’s ​​CPAK-800G​​ adapters via lane bonding
• ​​Co-Packaged Optics (CPO)​​: Acts a temporary interconnect during CPO migration phases

​​Final Perspective​​

Having benchmarked this cable in Tier IV data centers, its value lies not in raw speed but in ​​operational predictability​​. In environments where a single CRC error can cascade into GPU job failures, the QDD-4ZQ100-CU1M=’s BER performance and Cisco’s end-to-end signal integrity validation provide an assurance floor that third-party DACs can’t match. While Arista and Juniper offer similar specs, Cisco’s NX-OS integration—particularly with DCNM’s cable analytics—transforms this passive component into an active observability tool. For enterprises betting on 400G as the next decade’s backbone, it’s a tactical investment disguised as a commodity.