When discussing 100G ER4 modules, the conversation often starts and ends with distance. Forty kilometers becomes the headline feature, and design decisions are made around how far a link can reach. In real enterprise networks, however, optical module selection is far more closely tied to network layering than to raw transmission distance. Understanding where 100G ER4 fits within core and aggregation layers helps avoid overengineering and keeps networks efficient and manageable.
Where 100G ER4 Fits in a Layered Network
Modern enterprise networks are typically structured into access, aggregation, and core layers, each serving a distinct role. The access layer focuses on device connectivity and port density. The aggregation layer concentrates traffic and enforces policy. The core layer prioritizes high bandwidth, resiliency, and predictable performance.
100G ER4 naturally aligns with the core and upper aggregation layers, where fewer links carry large volumes of traffic between major network blocks. In these layers, bandwidth consolidation, link stability, and long-term scalability matter more than minimizing cost per port. ER4’s four-lane CWDM architecture and strong optical budget make it well suited for connecting core switches across campuses, metro sites, or large buildings without introducing additional optical complexity.
ER4 in the Core Layer: Stability Over Density
In the core layer, 100G ER4 serves as a straightforward, gray-optics solution that balances capacity and operational simplicity. Core links typically run between high-capacity switches or routers, where port count is limited but link reliability is critical. ER4 modules provide a clean 100G point-to-point connection without requiring external DWDM systems, amplification, or advanced tuning.
From a bandwidth planning perspective, ER4 allows core architects to standardize on a single 100G link type across different physical layouts. Whether the fiber run is a few kilometers or significantly longer, the same module type can be used, simplifying sparing strategies and reducing operational risk.
ER4 in the Aggregation Layer: Selective and Purpose-Driven
The aggregation layer is where ER4 must be used more selectively. Aggregation switches handle traffic from many access devices, but they also tend to have higher port density requirements. In this layer, 100G ER4 is most effective when used for uplinks toward the core, not for east-west connectivity between aggregation switches.
Using ER4 here makes sense when aggregation nodes are geographically distributed or when fiber routes are unpredictable. It provides enough optical margin to absorb real-world losses while maintaining consistent 100G capacity. However, using ER4 across every aggregation link can quickly inflate costs without delivering meaningful benefits over shorter-reach alternatives.
Why ER4 Is a Poor Fit for the Access Layer
At the access layer, the design priorities shift dramatically. Port density, power efficiency, and cost per connection dominate decision-making. Deploying 100G ER4 at this level offers little practical value. Most access links do not require the optical budget ER4 provides, and the module’s cost and power consumption are difficult to justify.
Additionally, access networks benefit more from scalable fan-out and short-reach optics, which align better with server, endpoint, and edge device connectivity. ER4’s role is not to reach endpoints, but to interconnect network layers where traffic converges.
Conclusion
100G ER4 should be viewed as a layer-aware building block, not a distance-driven solution. Its strength lies in supporting stable, high-capacity links between aggregation and core layers while keeping the optical architecture simple. When used with clear intent, ER4 helps networks scale cleanly without introducing unnecessary complexity at the edge.