Cisco SFP-10/25G-CSR-S: Dual-Rate 25G SFP28 Transceiver Explained

If you’ve spotted the part number SFP-10/25G-CSR-S while sourcing optics for a Cisco switch, it’s worth understanding exactly what “CSR” means here — it’s not a typo or a generic SR module, and its reach behaves differently depending on speed and FEC settings. Here’s what the module actually does, verified against Cisco’s official 25GBASE SFP28 datasheet.

What Is the SFP-10/25G-CSR-S?

The SFP-10/25G-CSR-S is a dual-rate SFP28 transceiver that operates at either 10 Gbps or 25 Gbps over multimode fiber, using an 850nm VCSEL laser and a duplex LC connector. “CSR” stands for Cisco’s extended short-reach optics — the same naming pattern used on the 40G CSR4 module, which similarly stretches beyond a standard SR module’s typical reach. In this case, that means considerably longer distance at 25G than Cisco’s standard SFP-25G-SR-S module offers.

Key Specifications

Reach Depends on Speed — And on FEC

This is the detail that trips people up most often: the SFP-10/25G-CSR-S doesn’t have one fixed reach number. It changes based on operating speed and, at 25G, on which FEC mode the host applies.

At 10G:

• 300m on OM3, 400m on OM4/OM5
• 82m on OM2
• No FEC required

At 25G:

• With RS-FEC (full reach): 300m on OM3, 400m on OM4/OM5; 70m on OM2
• With BASE-R FEC: 70m on OM3, 100m on OM4
• With no FEC: 30m on OM3, 50m on OM4

In other words, this module’s headline 300m/400m reach at 25G is only achieved with RS-FEC enabled on the host port. If your switch isn’t configured for RS-FEC, you’ll get noticeably shorter reach — worth checking before you assume the longer distance applies.

Why “Dual-Rate” Matters for Buyers

Because the SFP-10/25G-CSR-S runs at either 10G or 25G, it’s a practical choice for organizations in the middle of a 10G-to-25G migration. Rather than stocking separate 10G and 25G modules, the same part can serve both speeds — useful for phased upgrades where some links move to 25G before others, or as a standard spare that works regardless of which speed a given port needs.

A Note on Interoperability

Cisco’s datasheet flags a specific caution: when the SFP-10/25G-CSR-S interoperates with a standard IEEE 10G-SR module on the other end of a link, it may overload the receiver. Cisco directs buyers to its Optics-to-Optics Interoperability Matrix (tmgmatrix.cisco.com) to check supported pairings before mixing this module with third-party or non-Cisco 10G-SR optics on the same link.

Also worth noting: only patch cords with PC or UPC (physical contact/ultra-physical contact) connectors are supported. APC (angled physical contact) patch cords are not compatible with this module.

Where This Module Fits

The SFP-10/25G-CSR-S is a good fit for:

• Leaf-spine data center fabrics where switch-to-switch links may run 100–400m depending on rack layout
• Phased 10G-to-25G migrations, where a single dual-rate part simplifies sparing during the transition
• Inter-rack and aggregation links in cloud, enterprise, and service-provider environments where reach beyond a standard 25G SR module’s 70–100m is needed, but single-mode fiber isn’t installed

If your link is short (well under 70m) and you’re only ever running 25G, Cisco’s standard SFP-25G-SR-S may be a simpler, lower-cost choice. If you need 10km+ reach, this isn’t the right module at all — that’s the territory of the 10/25GBASE-LR SFP28 module instead.

Comparing the 25G SFP28 Options

Buying Considerations

Confirm your switch port’s FEC configuration before assuming maximum reach. The difference between 70m and 300m at 25G comes down entirely to whether RS-FEC is active on the host — check this before finalizing a cable run design.

Check OM3 vs. OM4/OM5 fiber grade for your actual installed cabling. Reach figures assume a specific fiber grade; running OM3 where OM4 reach was assumed will fall short.

Verify interoperability if mixing with non-Cisco 10G-SR optics. The receiver-overload caution in Cisco’s own datasheet is worth taking seriously — check the compatibility matrix rather than assuming it will work.

Use PC/UPC patch cords only. APC-terminated cables aren’t supported with this module — an easy detail to overlook when reusing existing patch cords from a different fiber project.

Consider the dual-rate flexibility if your migration timeline is uncertain. Paying slightly more for a dual-rate module can be cheaper long-term than needing to reorder if some links stay at 10G longer than planned.

Frequently Asked Questions

What does “CSR” mean in SFP-10/25G-CSR-S?
It refers to Cisco’s extended short-reach optics design — similar to the 40G CSR4 module — offering longer reach than a standard SR-class module at the same speed, when RS-FEC is enabled.

What’s the maximum reach of the SFP-10/25G-CSR-S?
Up to 400m on OM4/OM5 fiber, or 300m on OM3, but only at 25G with RS-FEC enabled on the host port. Without RS-FEC, reach at 25G drops significantly.

Does this module require FEC?
Not at 10G. At 25G, RS-FEC is required to achieve the module’s full 300m/400m reach; lower FEC modes or no FEC reduce the achievable distance.

Can I use this module for a 10km link?
No. It’s a multimode module with a maximum reach of 400m. For 10km reach at 10G or 25G, the SFP-10/25G-LR-S single-mode module is the correct choice.

Is the SFP-10/25G-CSR-S compatible with standard 10GBASE-SR modules on the other end of a link?
Not always without checking first — Cisco’s datasheet notes it may overload the receiver when interoperating with standard IEEE 10G-SR modules, and recommends checking Cisco’s Optics-to-Optics Interoperability Matrix before deploying that pairing.

Does it support FCoE?
Yes, with the appropriate host software support.

What connector type does it use?
A duplex LC/PC connector. Only PC or UPC patch cords are supported — APC connectors are not compatible.

Sourcing the SFP-10/25G-CSR-S

NextGen Networks stocks genuine Cisco SFP-10/25G-CSR-S modules alongside the broader Cisco 25GBASE SFP28 lineup, and can help confirm whether this dual-rate CSR module or a standard SR/LR variant best fits your specific link distance and FEC configuration.

Conclusion

The SFP-10/25G-CSR-S earns its place in Cisco’s lineup by combining dual-rate flexibility with meaningfully longer multimode reach than a standard 25G SR module — but only when RS-FEC is properly configured on the host side. Confirm your switch’s FEC setting, your installed fiber grade, and your patch cord connector type before ordering, and this module handles both current 10G links and future 25G upgrades from the same part number.