In Cisco’s 100G QSFP28 optical module product line, both the QSFP-100G-SR4-S and QSFP-100G-SL4 are multimode modules using the QSFP28 form factor and MPO-12 interface. However, they differ significantly in core performance and applicable scenarios. Understanding these differences is crucial for building efficient and reliable data center interconnects.

QSFP-100G-SR4-S: Short-Reach Up to 100m

• Technology: Based on the widely used 100GBASE-SR4 standard, it utilizes 850nm wavelength lasers (VCSELs).
• Official Specifications:

OM4 MMF: Up to 100 meters

OM3 MMF: Up to 70 meters

• Working Principle: Utilizes four independent channels (25 Gbps per channel) of multimode fiber for parallel transmission.
• Clarification of the meaning of “-S”: In Cisco nomenclature, “-S” does not denote “Extended” reach. Instead, it identifies a standard-power, compliant SR4 module, distinguishing it from certain proprietary enhanced or low-power models.
• Typical Applications: Providing industry-standard 100m (OM4) multimode transmission capabilities, it is a mainstream, universal choice for interconnecting data center cabinets (such as TOR-to-Leaf and Leaf-to-Spine) and equipment within the same equipment room. It offers high cost-effectiveness and a mature ecosystem.

QSFP-100G-SL4: Ultra-Short-Reach

• Technical Basis: Although its name may be confused with a single-mode module, the SL4 is still based on multimode fiber. It uses a 1310nm wavelength laser (typically an FP laser).

• Official Specifications:

OM4 MMF: Up to 30 meters

OM3 MMF: Up to 20 meters

• Technical Foundation: Based on the same 850nm VCSEL and 100GBASE-SR4 physical layer, but with extensive optimizations.
• “SL” stands for Short Link. Designed specifically for extremely short-reach applications.
• Core Advantages:

Ultra-Low Power Consumption: Significantly lower than standard SR4/SR4-S modules, typically around 1.5W or less.

Optimized Thermal Design: Low heat generation allows for higher port density deployments.

Optimized Electrical Characteristics: Signal integrity optimized for centimeter- to meter-level links (e.g., between card slots and on device backplanes).

• Typical Applications:

Interconnections between line cards within switch/router chassis (backplane patch cables).

Extremely short patch cables between adjacent cabinets or devices at the top of the same cabinet (e.g., between a TOR switch and a top-mounted patch panel).

Interconnections between devices within ultra-high-density server cabinets.

Any scenario requiring low power consumption and low heat generation with distances strictly less than 30 meters (OM4)/20 meters (OM3).

QSFP-100G-SR4-S vs. QSFP-100G-SL4, How to Choose?

Key Differences Summary:

Features	QSFP-100G-SR4-S	QSFP-100G-SL4
Wavelength	850nm (VCSEL)	850nm (VCSEL)
Maximum Reach (OM4)	100m	30m
Maximum Reach (OM3)	70m	20m
Standard Base	100GBASE-SR4 (Enhanced)	Physical layer compatible with SR4, optimized for short distances.
Key Advantages	Standard 100m transmission, strong versatility, and cost-effectiveness.	Extremely low power consumption (<1.5W typical), optimized for ultra-short distances.
Typical Scenarios	Inter-cabinet interconnections (≤100m OM4 / ≤70m OM3)	internal equipment/backplane patch cables, and extremely short connections between adjacent cabinets (≤30m OM4 / ≤20m OM3)

 

Selection Guide:

• Transmission distance requirements?

• > 30m (OM4) / > 20m (OM3) and ≤ 100m (OM4) / ≤ 70m (OM3): QSFP-100G-SR4-S (the only eligible option).
• ≤ 30m (OM4) / ≤ 20m (OM3): Proceed to the next evaluation step.

• Prioritized key requirements?

• Maximize port density & minimize power consumption/heat dissipation: QSFP-100G-SL4 (significant advantage).
• Compatibility/interchangeability with existing SR4 modules: QSFP-100G-SR4-S (more versatile).
• Cost-sensitive and very short links: The SL4’s lower power consumption may save electricity costs in the long term, but the initial module price difference needs to be evaluated.

Important Notes:

• Key considerations: Strictly adhere to distance limits: Exceeding the maximum distance specified for the module will result in link instability or complete failure.
• Fiber Type Matching: Both must use OM3 or OM4 multimode fiber. Single-mode fiber is incompatible.
• Wavelength Matching: Physically interoperable, but logically different: Although both use 850nm and MPO-12 interfaces and can physically plug into the same port, the SL4 is a specialized model optimized for extremely short distances. Both ends of the link must use the same module model (i.e., SR4-S with SR4-S, SL4 with SL4). Mixing modules may result in negotiation failures or poor performance.
• “-S” designation: Cisco documentation clearly indicates distances of OM4-100m/OM3-70m. Do not misunderstand this.

Conclusion:

The QSFP-100G-SR4-S is a cornerstone module compliant with the 100GBASE-SR4 standard, meeting the requirements for universal short-reach connections of 70m (OM3)/100m (OM4). The QSFP-100G-SL4 is a low-power Cisco product specifically designed for extremely short links of less than 30 meters (OM4) or 20 meters (OM3). It offers irreplaceable energy-saving and heat dissipation advantages for internal device interconnects and high-density rack deployments. The key to selection is accurate measurement of physical distances and identification of the application’s core requirements (standard versatility vs. ultra-low power consumption), while also ensuring strict consistency of module models at both ends of the link.