As enterprises upgrade their networks from 100G to 400G, they face a critical challenge: how to maintain service continuity while phasing in new high-speed optics. This article provides a comprehensive migration playbook covering the most widely deployed 100G modules – QSFP28 100G LR4, ER4, ZR4, QSFP28 100G 100KM, QSFP28 100G BIDI 40KM, and QSFP28 100G BIDI 80KM – and their 400G successors: OSFP112-400G-VSR4, QSFP56-DD-400G-VSR4, QSFP DD DR4, and QSFP112. We will examine port compatibility, breakout modes, hybrid link designs, and common pitfalls to ensure a smooth transition.
Before planning a migration, network teams must inventory their existing 100G infrastructure. The dominant package is QSFP28, which supports a wide range of reaches via different optics.
The QSFP28 100G LR4 is the most common module for links up to 10km, using four LAN-WDM wavelengths. For distances up to 40km, QSFP28 100G ER4 adds an APD receiver and SOA. For 80km, QSFP28 100G ZR4 provides higher sensitivity and stronger FEC. The QSFP28 100G 100KM segment is specialized – typically using coherent or semi-coherent technology, often requiring external amplification. These modules all use duplex LC connectors and are designed for single-mode fiber.
When fiber pairs are scarce, QSFP28 100G BIDI 40KM and QSFP28 100G BIDI 80KM provide 100G over a single fiber using bidirectional wavelength multiplexing. They maintain duplex LC connectors but internally separate TX and RX paths via WDM filters. These BIDI modules are not interoperable with standard LR4/ER4/ZR4 modules – both ends of a link must be BIDI type.
400G modules come in three major physical packages, each with different backward compatibility characteristics.
OSFP112-400G-VSR4 belongs to the OSFP family, which is physically larger than QSFP-DD but offers better thermal management. OSFP modules are not directly pluggable into QSFP28 or QSFP-DD cages. However, many OSFP switches support breakout cables that convert one OSFP port into four QSFP28 100G ports, enabling a hybrid migration. The VSR4 variant is optimized for ≤2km links with low power (~9W).
QSFP-DD (QDD) is the most popular 400G package due to its backward compatibility with QSFP28 cages. A QSFP-DD port can accept a QSFP28 module (100G) natively, though the reverse is not true. QSFP56-DD-400G-VSR4 and QSFP DD DR4 are two key optical variants: VSR4 for 2km, DR4 for 500m. The QSFP112 (not to be confused with QSFP28) is a different 400G package that retains the QSFP outer dimensions but uses 4×112G electricals. It is less common than QSFP-DD but offers a direct upgrade path for existing QSFP28-based line cards that support 112G per lane.
Based on the existing 100G deployment, three primary migration strategies exist: port reuse, breakout migration, and fiber overlay.
If you are deploying new 400G switches with QSFP-DD cages, you can initially populate them with existing QSFP28 100G LR4 or ER4 modules while waiting for 400G optics. This allows a “pay-as-you-grow” model. Once traffic demands justify it, replace the 100G modules with QSFP DD DR4 (for short links) or QSFP56-DD-400G-VSR4 (for 2km links). No fiber re-cabling is required because both 100G LR4 and 400G DR4 use duplex LC (though DR4 typically uses MPO – careful: DR4 actually uses MPO-12, not LC. This is a common pitfall. Note: QSFP DD DR4 uses MPO-12 connectors, while QSFP28 LR4 uses duplex LC. Therefore, a fiber plant designed for LR4 cannot directly accept DR4 without changing patch panels or using conversion cables. This is critical.)
Correction for accuracy: QSFP DD DR4 requires MPO-12 (APC) to four duplex LC breakout cables if connecting to 100G LR4 optics. Alternatively, use 400G-FR4 (which uses duplex LC) for a smoother migration, but FR4 is not in our keyword list. For strict adherence to given keywords, if you have existing LR4 fibers (duplex LC), you cannot directly plug a DR4 module. You would need a DR4 to 4x100G breakout cable – but that would break out to four separate 100G LR4 modules, not a single 400G link. Therefore, the simplest migration from duplex LC 100G is to use QSFP56-DD-400G-VSR4 (which often supports duplex LC, check datasheet) or to re-terminate the fiber plant to MPO.
Many 400G switches support port breakout: a single OSFP112-400G-VSR4 or QSFP56-DD-400G-VSR4 can be connected via a fanout cable to four separate 100G QSFP28 modules. This is ideal for gradually upgrading a leaf-spine topology. For example, you can keep your existing 100G servers and replace the spine switch with a 400G model. Use breakout cables to connect the 400G spine ports to four leaf switches, each running QSFP28 100G LR4. When you later upgrade the leaf switches to 400G, you can switch to direct 400G links. This strategy works well with VSR4 modules because they have lower power and are designed for short reaches where breakout cables are feasible.
If your existing 100G links use QSFP28 100G BIDI 40KM or BIDI 80KM on a single fiber, you can often reuse that same fiber for 400G using OSFP112-400G-VSR4 or QSFP56-DD-400G-VSR4 provided that the 400G module also supports bidirectional transmission. Not all VSR4 modules are BIDI-capable; most are dual-fiber. However, some vendors offer BIDI versions of VSR4. When migrating, verify the wavelength plan: 100G BIDI typically uses 1270/1330nm or 1290/1310nm; 400G BIDI may use different pairs (e.g., 1310/1270nm for 2x200G). Interoperability between 100G BIDI and 400G BIDI on the same fiber is not possible because speeds and modulations differ. But you can repurpose the dark fiber for a new 400G BIDI link while decommissioning the 100G link.
The following table summarizes which modules can directly connect or interoperate via breakout.
QSFP28 100G LR4 ↔ QSFP28 100G LR4 (same type required). Cannot directly connect to any 400G module.
QSFP28 100G BIDI 40KM ↔ QSFP28 100G BIDI 40KM (both ends BIDI). Not compatible with LR4/ER4.
QSFP DD DR4 (MPO-12) ↔ QSFP DD DR4 (MPO-12). To connect to four 100G LR4 modules, use a DR4 breakout cable (MPO-12 to 4x duplex LC). Each 100G LR4 sees a standard 100G signal.
QSFP56-DD-400G-VSR4 (duplex LC or MPO – check vendor) ↔ same type. Some VSR4 modules use duplex LC for 2km SMF, making them drop-in replacements for LR4 fibers (but at 400G speed). Ensure the fiber length is ≤2km.
OSFP112-400G-VSR4 ↔ QSFP56-DD-400G-VSR4 cannot directly connect because OSFP and QSFP-DD cages differ, but the optical signals can interoperate if both are VSR4 with same wavelength and FEC. You would need an OSFP-to-QSFP-DD adapter or cable.
Critical pitfall: Many engineers assume that because QSFP-DD ports accept QSFP28 modules, the reverse is true – it is not. You cannot plug a 400G module into a 100G-only QSFP28 cage. The electrical lanes and firmware are incompatible.
Consider a metro ring with five nodes, each connected by 80km spans using QSFP28 100G ZR4 modules on duplex LC fibers. Traffic growth requires upgrading two high-traffic spans to 400G while leaving others at 100G.
Solution: On the two spans, replace the 100G ZR4 modules with QSFP56-DD-400G-VSR4 modules – but VSR4 only supports 2km, not 80km. So that won’t work. Actually, for 80km, you need a 400G long-haul module like 400G-LR4 (10km) or 400G-ER4 (40km) or coherent 400G-ZR (80km+). However, those are not in our keyword list. Given keyword constraints, the realistic migration for 80km is to stick with 100G ZR4 and aggregate multiple 100G links via bonding, or upgrade to a coherent 400G solution (e.g., 400G-ZR in QSFP-DD, not listed). Therefore, we must be honest: OSFP112-400G-VSR4 and QSFP56-DD-400G-VSR4 are not suitable for 80km. This highlights an important principle: VSR4 is for short-reach only. For long-haul migration, you would need different 400G optics. So in practice, many enterprises maintain 100G for long spans and deploy 400G only for short spans.
A: Not directly. LR4 uses duplex LC connectors; DR4 uses MPO-12. You can use an MPO-12 to 4x duplex LC breakout cable, but that converts the 400G DR4 into four separate 100G LR4 connections, not a single 400G link. For a native 400G link, you need to re-terminate the fiber to MPO-12 or use a 400G module with duplex LC (e.g., 400G-FR4, not in this list).
A: No. BIDI uses single-fiber bidirectional transmission; ER4 uses dual-fiber. They are not interoperable. Both ends must be BIDI type with matching wavelengths.
A: Typically 2km on single-mode fiber. Some variants may support up to 4km with premium fiber and FEC, but always check the data sheet. Do not use VSR4 for 10km or longer spans.
A: No. QSFP112 has the same mechanical dimensions but different electrical signaling (112G PAM4 vs 25G NRZ) and firmware. It will not be recognized by a legacy QSFP28 port.
A: They cannot directly interoperate because speeds differ (400G vs 100G). You would need a rate-conversion device (e.g., a switch that breaks out 400G to 4x100G). The VSR4 module would connect to the switch, and the switch’s 100G BIDI ports would connect to the existing link.
A: Most modern QSFP DD DR4 and OSFP112-400G-VSR4 support Clause 91 RS-FEC (544,514) as defined by IEEE 802.3bs. However, FEC settings must match on both ends. Some vendors implement proprietary FEC – avoid those for multi-vendor interoperability.
Migrating from 100G (QSFP28 LR4/ER4/ZR4/100KM/BIDI 40KM/BIDI 80KM) to 400G (OSFP112-400G-VSR4, QSFP56-DD-400G-VSR4, QSFP DD DR4, QSFP112) requires careful planning of connectors, fiber types, reach limitations, and breakout modes. Our team provides free migration audits, including on-site fiber plant assessment and compatibility testing.
Contact us today to schedule a consultation. We offer trade-in programs for legacy 100G modules and volume discounts on 400G optics. Let us help you build a seamless, high-performance network for the next decade.
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