What are SFP, SFP+, QSFP and DAC, and what should you check before plugging one in?
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They are hot-swappable pluggable modules that give a port its physical layer. SFP is 1 Gbps, SFP+ 10 Gbps, SFP28 25 Gbps, QSFP+ 40 Gbps (4 x 10), QSFP28 100 Gbps (4 x 25), and QSFP-DD or OSFP carry 400 and 800 Gbps. A DAC is twinax copper with modules fixed on each end, cheap for a few meters in a rack; an AOC is the same idea with fiber. Before plugging in, check the platform compatibility list, matching optics on both ends, the fiber type and the reach.
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Form factors
| Module | Speed | Lanes | Typical connector |
|---|---|---|---|
| SFP | 1 Gbps (also 100BASE-FX) | 1 | Duplex LC or RJ-45 |
| SFP+ | 10 Gbps | 1 | Duplex LC |
| SFP28 | 25 Gbps | 1 | Duplex LC |
| QSFP+ | 40 Gbps | 4 x 10G | MPO or duplex LC (LR4) |
| QSFP28 | 100 Gbps | 4 x 25G | MPO or duplex LC |
| QSFP-DD, OSFP | 400 to 800 Gbps | 8 lanes | MPO or duplex LC |
Things that bite
- Speed support: an SFP+ port often accepts a 1G SFP, but an SFP port never runs an SFP+ at 10G. Confirm per platform.
- Breakout: a 40G or 100G port can split into four 10G or 25G ports with a breakout cable, if the platform supports it and you configure it.
- Vendor coding: switches read the module's EEPROM. Cisco IOS rejects unsupported modules and can
err-disable the port with reason
gbic-invalid. - Both ends must match: SR to SR, LR to LR. Different reach classes do not interoperate reliably.
- DOM: digital optical monitoring reports temperature, voltage, laser bias, Tx and Rx power.
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They will ask next
Why use a DAC instead of two optics and a patch cord?
Cost and power. A passive DAC needs almost no power and costs a fraction of two optics. The limit is reach (a few meters passive, longer with active DAC) and thick, stiff cables.
Can a 10GBASE-SR on one side talk to a 10GBASE-LR on the other?
No. SR uses 850 nm on multimode and LR uses 1310 nm on single-mode. Both ends need the same type.
Traps
- Assuming any module fits any port of the same shape. Check speed, platform support and coding.