Cut Sheet

Table Of Contents

COTB02--WW-ENG, Rev.0, 10/2014 Page 1 of 2

25 Gigabit Ethernet for Servers: What Does it Mean?

Introduction

Until recently, the Ethernet speed upgrade path was clearly defined as 10G  40G  100G. However,

new developments indicate the latest path for server connections will be 10G  25G 100G with

potential for 10G  25G 50G  100G. These developments are being driven by companies such as

Cisco, Microsoft, and Intel because 25G provides a more efficient use of hardware and a more logical

upgrade path to 100G. 25G Ethernet is also in the process of becoming a standard and has passed the

first hurdle in the IEEE standards body with a successful Call for Interest (CFI) in July, 2014. The industry

expects that 25G hardware will be available as early as 2015 with the standard finalized by 2016.

40 Gigabit to the Server Will be Limited

While it is impossible to see the future, the current projections by switch and server analysts show that

25G will limit the deployment of QSFP+ 40G ports at the server level. While switch-to-switch connections

will stay at 40G, expect that the dominant next generation server speed will be 25G. As a result, Panduit

recommends that data centers be planned with 25G at the server. Subsequent upgrade paths will likely

include an optional step at 50G and the following step at 100G.

QSFP28 to SFP28 Breakout Cables for Top of Rack with Up to Five Meters of Reach

The IEEE CFI is focused on the Direct Attach Copper (DAC) which includes QSFP28 (based on the IEEE

802.3bj 100G standard) and SFP28 (will be defined by the new IEEE study group). QSFP28 refers to the

100G DAC cable using the QSFP+ form factor, and SFP28 refers to the 25G DAC cable using the SFP+

form factor. While the form factor remains the same, the new standard will require improved cables and

connectors capable of handling this additional bandwidth. Existing QSFP+ and SFP+ cable assemblies

will not be compatible with QSFP28 and SFP28 ports.

The goal of the IEEE CFI is to use 25G DAC as the low-cost copper server connection for Top of Rack

(ToR) switches. Reach is expected to be about the same as existing passive DAC cables or about five

meters. There is speculation that some equipment may only run at three meters, but it is still too early in

the process to validate or verify that possibility

Currently the cabling application is one QSFP28 to four SFP28 cables. The

target application for the CFI is 100G switches connecting to four 25 Gigabit

in cabinet or adjacent cabinet servers using these breakout cables. There is

also expected to be SFP28 to SFP28 cabling, assuming equipment vendors

develop a ToR 25G switch. A 25G fiber option is also being developed based

on duplex LC that would mate to the SFP28 ports.

Figure 1: QSFP28 to four

SFP28 breakout cable

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25 Gigabit Ethernet for Servers: What Does it Mean? Introduction Until recently, the Ethernet speed upgrade path was clearly defined as 10G  40G  100G. However, new developments indicate the latest path for server connections will be 10G  25G 100G with potential for 10G  25G 50G  100G. These developments are being driven by companies such as Cisco, Microsoft, and Intel because 25G provides a more efficient use of hardware and a more logical upgrade path to 100G.
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40GBASE-T will Include 25GBASE-T and Use the Same Category 8 Channel Already Developed Because the industry-wide view is that 40G to the server will be limited in scope, the development of the 40GBASE-T standard (IEEE 802.3bq) will expand to include 25GBASE-T to support 25G to the server over twisted pair. The plan is to have a new Call for Interest in the IEEE to officially begin work on 25GBASE-T in November, 2014. It will then roll into 40GBASE-T to become the 25G/40GBASE-T standard.