QSFP28 100G: Plays with 40G, 25G and 10G

Updated on Mar 8, 2022 by

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Among the transceivers that are capable of handling 100G ethernet traffic, the compact QSFP28 transceiver increases data center density, being the main data center form factor in the 100G gigabit ethernet era. Is QSFP28 compatible with QSFP+? Can I use the QSFP+ optics on the QSFP28 port? Whether it is possible to insert QSFP+ optics on QSFP28 ports to support 10G SFP+? Here we look at how the QSFP28 100G port can work at speeds other than 100Gbps. The configuration commands and hardware needed will be covered in this article illustrated with FS 100G switches and transceivers.

How to Run QSFP+ 40Gbps on QSFP28 100G Port?

If the opposite equipment leverages 40G service speed, the 100G QSFP28 interface should speed down to the corresponding 40G and work with 40G QSFP+ optics.

Hardware Support

QSFP+ 40G transceiver with MTP/MPO connector; MTP/MPO 12 Fiber Trunk Cable
40G QSFP+ to QSFP+ DAC Cable
40G QSFP+ to QSFP+ AOC Cable

If you use the QSFP transceivers in the QSFP28 100G port, keep in mind that you have both single-mode and multimode (SR/LR) optical transceivers and Twinax/AOC options that are available.

QSFP+ 40G on QSFP28 Configuration

QSFP28 100G transceivers have the same form factor as the QSFP optical transceiver. The former has just 4 electrical lanes that can be used as a 4x 25GBE and also a 4x 10GbE, while the latter supports 40G ( 4x 10G). Here takes the QSFP 100G port on FS S5850-48S2Q4C LAN distribution switch as an example to accommodate QSFP transceiver carrying 40GbE traffic.

S5850-48S2Q4C# configure terminal

S5850-48S2Q4C(config)# interface eth-0-49

S5850-48S2Q4C(config-if)# speed 40G

How to Realize 4x 10GbE Mode on QSFP28 100G Port?

It is viable to run the 100G QSFP28 port into 4x 10GbE mode using a QSFP+ 40G optical transceiver when the new switch on the one side only possesses 100G ports and the existing devices on the other side only have SFP+ 10G ports.

Hardware Support

QSFP+ 40G transceiver with MTP/MPO connector; SFP+ 10G transceivers with LC connector; MTP/MPO-LC harness cable
40G QSFP+ to 4x 10G SFP+ Passive DAC Breakout Cable
40G QSFP+ to 4x 10G SFP+ Breakout AOC Cable

SFP+ 10Gbps on QSFP28 Configuration

The commands first change the speed of the interface 49 (100G port) on FS S5850-48S2Q4C into 40GbE and then break it out into 4x 10GbE mode.

S5850-48S2Q4C# configure terminal

S5850-48S2Q4C(config)# interface eth-0-49

S5850-48S2Q4C(config-if)# speed 40G

S5850-48S2Q4C(config)# split interface eth-0-49 10gigabit

How to Realize 4x 25GbE Mode on QSFP28 100G Port?

Four SFP28 can make full use of the electrical lanes of QSFP28 to realize 100G transmission. Here is illustrated FS N8500-32C which is a 100Gb Spine/Core Layer Switch with PicOS®.

Hardware Support

QSFP28 100G transceiver with MTP/MPO connector; SFP28 25G transceivers with LC connector; MTP/MPO-LC harness cable
100G QSFP28 to 4x 25G SFP28 Passive DAC Breakout Cable
100G QSFP28 to 4x 25G SFP28 Breakout AOC Cable

SFP28 25G on QSFP28 Configuration

The following commands make it possible to step back to the default mode of the QSFP28 100G port.

Disable the function by typing “no” before the command

eg:

S5850-48S2Q4C(config)# split interface eth-0-33 25gigabit

S5850-48S2Q4C(config)# no split interface eth-0-33 25gigabit

Restore factory settings

Switch// delete Startup-config

Conclusion

Aside from the 100G mode, the QSFP28 100G port on the switch can also run in 40GbE, 4x 25GbE and 4x 10GbE mode, with the required hardware and configuration commands exemplified by the FS S5850 48S2Q4C. Besides, the way to get back to the default mode of the switch port is of equal importance to the process. FS provides comprehensive data center products and solutions to build your data center.