Networking Basics
SFP, SFP+, single-mode vs multi-mode, BiDi and DAC explained — how to specify the right optic for a fibre uplink and avoid the usual compatibility traps.
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An SFP is a Small Form-factor Pluggable transceiver — a removable module that turns a switch port into whatever media the link needs. The same physical cage can take a short multi-mode optic, a long single-mode optic, or a copper direct-attach cable.
That modularity is the point: the switch is bought once, and the optics are chosen per link and replaced independently when a run changes.
Three attributes define any module: speed, fibre type and reach, and the connector or cable arrangement.
SFP and SFP+ modules are the same size and fit the same slot, but SFP+ is specified for 10 Gbps. A 10G-capable port will normally accept a 1G module for a slower link; the reverse is not true.
In the Lysora range the LS2 switches provide 1GE SFP slots for uplinks, while the LS2P-8MG2XS-P offers 10GE SFP+ and the LS3-24SFP/8GT4XS combines SFP with SFP+ for fibre aggregation.
The optic and the fibre must match — this is the most common cause of a link that will not come up.
BiDi (bidirectional) modules send and receive on a single strand using different wavelengths, which halves the fibre needed on a constrained route. They are always deployed as a matched pair, one of each wavelength, at opposite ends.
Direct Attach Copper (DAC) is a fixed-length copper cable with transceiver ends moulded on. For rack-to-rack links of a few metres it is cheaper and lower-power than optics, but it cannot be re-terminated and does not span distance.
Many switch vendors expect transceivers to carry particular identification data, and some reject modules that do not match. This is a commercial rather than an optical constraint, but it causes real deployment delays.
The practical protection is to confirm the intended switch model when ordering optics, so the modules supplied are known to be accepted by that platform.
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Before You Enquire
Usually yes — a 10G-capable port will typically run a 1G module, and the link negotiates at 1 Gbps. The opposite does not work: a 1G SFP port cannot drive an SFP+ module at 10 Gbps.
They must be compatible in speed, wavelength, and fibre type. Standard duplex optics are normally identical at both ends, whereas BiDi optics are deliberately a matched pair of two different wavelengths.
The LS2 managed switches provide 1GE SFP uplink slots, the LS2P-8MG2XS-P adds 2× 10GE SFP+, and the LS3-24SFP/8GT4XS is built around 24 SFP ports with SFP+ uplinks for fibre aggregation.
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