SFP/QSFP Transceiver Buyer's Guide

Form Factor and Speed — SFP, SFP+, SFP28, QSFP+, QSFP28, QSFP-DD

SFP is the original 1G transceiver form factor. Still in service on legacy switches and access-layer gear.

SFP+ supports 10G. Dominant in modern leaf switches for server-facing ports.

SFP28 supports 25G. The current sweet spot for new server uplinks; replacing 10G in greenfield builds.

QSFP+ supports 40G (4x 10G aggregated). Common on legacy spine ports; being replaced by 100G QSFP28 in new deployments.

QSFP28 supports 100G (4x 25G). Dominant for current spine-leaf uplinks.

QSFP-DD supports 400G (8x 50G). Top-tier for hyperscale and high-end enterprise spines. Backward-compatible with QSFP28 in some chassis (single-mode operation).

Match form factor to chassis port specification — you cannot insert QSFP+ into SFP+ slots or vice versa.

Fiber Type — MMF, SMF, DAC, AOC

DAC (Direct-Attach Copper) is passive copper cable with optics integrated at both ends. Best for short runs (1-7m), lowest cost, lowest latency. Works at 10G / 25G / 100G / 400G.

AOC (Active Optical Cable) uses multimode fiber internally with permanently-attached optics. Best for 7-30m runs at 10G / 25G / 100G. Lighter than copper, easier cable management at scale.

MMF (Multimode Fiber) with SR (Short Reach) optics supports 100-150m at 10G / 25G / 100G depending on fiber grade. OM3 = 100m at 10G, 70m at 25G; OM4 = 150m at 10G, 100m at 25G; OM5 = 300m at 10G with bidi optics.

SMF (Single-Mode Fiber) with LR optics supports 10km. Required for inter-building runs, DCI, and any application beyond MMF range. SR4 = 100m, LR4 = 10km, ER4 = 40km, ZR4 = 80km coherent.

Vendor Coding — Why Cisco Won't Take Arista Optics

Optical transceivers are physically interoperable across vendors but logically restricted by firmware coding. The optic has an EEPROM that identifies its vendor; the switch matches against an allowed list.

For Cisco IOS / IOS-XE / NX-OS switches, the optic must be coded for Cisco. Non-Cisco coding produces "service unsupported-transceiver" warnings.

For Arista EOS, the optic must be coded for Arista. Same warning pattern.

For HPE Aruba ArubaOS-CX, code for HPE/Aruba.

For Juniper Junos, code for Juniper.

Pro Disk Network ships every transceiver pre-coded for the target vendor. Specify the chassis vendor when ordering. Mixing optics across vendor codings in the same chassis sometimes works but logs warnings.

Distance Planning — Get the Optic Right the First Time

Mismatched distance ratings produce intermittent link errors that look like cable problems but aren't. SR4 optic on an 8km run fails to link or links unstably. LR4 optic on a 50m run can saturate the receiver and produce CRC errors.

Document the cable plant before ordering optics. Measure cable lengths between racks; note which racks have MMF vs SMF.

For greenfield builds, prefer SMF in new conduits even for short runs. Marginal cost is small; the future-proofing for 100G / 400G upgrades is meaningful.

Pre-Shipment Validation

For any large transceiver order (50+ units), request a pre-shipment validation report. Pro Disk Network can run TX/RX power level checks on each module before dispatch and ship a CSV with the test data.

For DDM/DOM-capable optics, run `show interfaces transceiver` on the switch after installation. Healthy optics show TX power in the spec's middle third (e.g., -2 to -5 dBm for SFP+) and RX power well above the receiver threshold (typically -15 to -19 dBm minimum). Drift outside spec indicates a failing optic or a contaminated connector.

For long-haul ZR4 / CFP4 modules, export-control screening may apply. Confirm destination country and end-use before order placement.