What the standard defines
IEEE 802.3bz defined two physical layers at once: 2.5GBASE-T and 5GBASE-T, sitting between gigabit and ten-gigabit operation over four-pair balanced copper.
The design constraint was the cabling already in buildings. Ten-gigabit copper needs a Category 6A channel to reach a hundred metres, and pulling new cable through a finished wall is the expensive part of any upgrade. So the amendment specifies 2.5GBASE-T over a Category 5e channel at the full hundred metres, and 5GBASE-T over a Category 6 channel at the full hundred metres. The point of the rates is that the cabling does not have to change.
Ports negotiate as they always have. A multi-rate copper port advertises the set of rates it supports and settles on the highest one shared with the far end, which is why the same socket may come up at a gigabit with one device and at two-and-a-half with another.
Before the amendment was published, equipment appeared under the trade name NBASE-T; the designation to look for on a data sheet is the IEEE one.
What it does not promise
It does not promise two and a half times the transfer rate. This is a line rate, and a copy still finishes at the speed of the slowest thing in the path — commonly a hard disk, an encrypted share, or a USB bridge in an enclosure.
It does not promise anything about the connection to the outside world. A router with a two-and-a-half gigabit socket on the line side can only carry what the line delivers.
It does not work with one end. A multi-gig port facing a gigabit adapter negotiates a gigabit, and the upgrade has to be made twice — at the switch and at the machine.
It does not come free of heat. Running above a gigabit over copper takes more power and produces more warmth than gigabit operation, which is a real constraint on small fanless switches; the consumption a maker publishes is the figure to read.
And it does not repair a channel. The amendment names cabling grades because it depends on them, and a link that will not come up at the higher rate is usually reporting on the cable rather than on the switch.
What it works alongside
It negotiates down to gigabit and below, so nothing older stops working. It runs over Category 5e, 6 and 6A channels, and appears on multi-gig ports that also offer ten-gigabit operation. Power over Ethernet travels across the same pairs, which is why access points that want more than a gigabit of uplink and their power from the same cable are the commonest place to meet it.
The questions that come up before an order
Does a two-and-a-half gigabit port need new cabling?
The amendment was specified to run over a Category 5e channel at the full hundred metres, which is why it exists. A channel that measures to its grade should carry it; one that was marginal at a gigabit is the case where the rate will not come up.
What is NBASE-T?
The name products used before the amendment was published. Equipment from that period generally interoperates with the standardised rates, but the guarantee comes from the IEEE designation on the data sheet rather than from the earlier trade name.
Last reviewed 9 September 2026