Because 25 Gb is one lane and 40 Gb is four - so 25 Gb is cheaper, simpler and uses far less fibre for the bandwidth it delivers.

40 Gb was built as four 10 Gb lanes in a quad cage, needing either eight fibres in a parallel ribbon or wavelength multiplexing, and a larger, more expensive module.

When 100 Gb moved to four lanes of 25 Gb, a single 25 Gb lane became a commodity. One of those in an SFP-sized cage gives 25 Gb over one fibre pair, at a module cost and port density close to 10 Gb.

The economics follow: 25 Gb delivers two and a half times the bandwidth of 10 Gb for a modest premium, uses the same cabling, and fits the same faceplate.

It also lines up with the uplinks. A 100 Gb switch port breaks out into four 25 Gb connections, so the access and uplink speeds are multiples of the same lane rate - which is why 25/100 displaced 10/40 as the standard data centre pairing.

A coding scheme that corrects bit errors in transit - and at 25 Gb per lane it is part of the link rather than an optional extra, which makes mismatches a common fault.

As lane rates rise, the raw error rate before correction rises with them. At 25 Gb the signal is expected to arrive with errors that the correction removes, so the link is engineered around it.

There are several modes - none, a lighter scheme, and a stronger one - and which is required depends on the media and reach. Direct attach cables and short optical links may need little or none; longer links need the stronger mode.

Both ends must agree. Where one device defaults to one mode and the other to another, the link either fails to establish or comes up with a high error rate, and nothing about the optics is wrong.

So when a 25 Gb link misbehaves, check the correction setting on both ports early. It is the single most common cause and it is invisible from the optical diagnostics.

Generally yes - SFP28 ports are usually backwards compatible with 10 Gb and often 1 Gb modules, which makes migration straightforward.

The cage and the electrical interface are the same family, so a 25 Gb port typically accepts a 10 Gb module and runs the port at 10 Gb. That allows a new switch to be deployed with existing 10 Gb optics and servers, and for individual ports to be moved to 25 Gb as hosts are upgraded.

Gigabit support is more variable and should be checked per platform.

The speed usually has to be set or is derived from the module, and some platforms group ports so that speed changes affect a group rather than a single port.

The reverse does not work: an SFP28 module in an SFP+ port will not run at 25 Gb.

For a phased upgrade, this compatibility is the main practical benefit - the switch can be replaced first and the hosts afterwards, without a flag day.

The same fibre types as 10 Gb, but with tighter distance limits on multimode - and for in-rack connections, direct attach copper.

Single-mode links use a duplex pair exactly as at 10 Gb, and the reach is comparable.

Multimode is where the difference shows. Higher lane rates are more sensitive to modal dispersion, so the supported distance is shorter than the equivalent 10 Gb link on the same fibre grade. Older multimode grades in particular may not support 25 Gb over the distances they carried at 10 Gb.

Check the fibre grade and the actual length against the module's specification. This catches people out on building backbones installed years earlier for 10 Gb.

Within a rack, direct attach copper is the normal choice for server connections - cheapest, lowest power and lowest latency - with active optical cables where the distance exceeds what passive copper manages.

For new deployments usually yes, because the price difference is small relative to the capability and it aligns with 100 Gb uplinks.

The cost per port for 25 Gb is modestly above 10 Gb while delivering two and a half times the bandwidth, so the cost per gigabit is substantially better.

It also matches the uplink generation. A top-of-rack switch with 25 Gb access ports and 100 Gb uplinks has a clean 4:1 relationship, and breakout cables let one uplink port serve four access connections.

Where it does not pay is where the servers cannot use it. A workload that never exceeds a gigabit gains nothing from 25 Gb, and the network card, the PCIe slot and the application all have to be capable of the throughput before the link speed matters.

For virtualisation hosts, storage nodes and anything with heavy east-west traffic, 25 Gb is now the sensible default. For general application servers, it is worth specifying on new hardware simply because the marginal cost is low.

Somewhat, and in dense deployments the aggregate matters more than any single module.

Higher lane rates need more sophisticated electronics, and a 25 Gb module typically dissipates more than a 10 Gb equivalent - not dramatically, but a fully populated 48-port switch multiplies the difference.

The practical consequences are about the switch rather than the module: confirm the platform supports full population with the module type intended, since some high-density switches specify limits on how many high-power modules may be fitted, and check the airflow direction and cabinet ventilation.

Modules report their own temperature through digital diagnostics, and reading those figures across a populated switch quickly reveals whether the thermal design is adequate - the modules at the hot end of the chassis will show it.

Direct attach copper cables dissipate far less than optical modules, which is among the reasons they are preferred for in-rack connections at scale.

A 100 Gb quad port is configured to operate as four independent 25 Gb ports, connected by a breakout cable to four SFP28 ports.

A QSFP28 port carries four 25 Gb lanes. In breakout mode the switch presents them as four separate interfaces, each with its own configuration, and a splitter cable takes the four lanes to four separate connections.

The cable may be direct attach copper splitting into four SFP28 ends, an active optical equivalent, or a parallel fibre assembly with a fan-out to four duplex connections.

The benefit is density and cost: one physical port on an aggregation switch serves four servers or four small switches, at less cost than four individual ports.

The constraints are that the port must support the mode - not all do, and some restrict it to certain ports - and that all four sub-ports are tied to one physical connection, so a single cable fault takes down four links. That last point matters when planning redundancy.