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

Posted on October 25, 2019
July 6, 2020
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Among the transceivers that are capable of handling 100G traffic, the compact QSFP28 transceiver increases data center density, with lower power consumption and price per bit, going to be the main data center form factor in 100G era. Is QSFP28 compatible with QSFP+? Can I use the QSFP+ optics on QSFP28 port? Whether it is possible to insert QSFP+ optics on QSFP28 ports to support 10G SFP+? Well, all these puzzles center on the application of the QSFP28 port on the network switch. Here we look at three situations where the QSFP28 100G port works at other speeds than 100Gbps. The configuration commands and hardware needed will be covered in this article illustrated with FS 100G network switches and 100G transceivers.


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How to Run 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 Fibers Trunk Cable

  • 40G QSFP+ to QSFP+ DAC Cable

  • 40G QSFP+ to QSFP+ AOC Cable

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

Configuration

QSFP28 transceivers have the same form factor as the QSFP optical transceiver. The former has just 4 electrical lanes that can be used as a 4x25GBE and also a 4x10GbE, while the latter supports 40G ( 4x10G). Here takes the 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 4*10GbE Mode on QSFP28 100G Port

It is viable to run the QSFP28 port into 4*10GbE mode using 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 4x10G SFP+ Passive DAC Breakout Cable

  • 40G QSFP+ to 4x10G SFP+ Breakout AOC Cable

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 4*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 4*25GbE Mode on QSFP28 100G Port

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

Hardware Support

  • QSFP28 100G transceiver with MTP/MPO connector; SFP28 25G transceivers with LC connector; MTP/MPO-LC harness cable

  • 100G QSFP28 to 4x25G SFP28 Passive DAC Breakout Cable

  • 100G QSFP28 to 4x25G SFP28 Breakout AOC Cable

Configuration

The commands split the interface 32 (100G port) on FS N8500-32C switch into 4*25GbE mode.

root@cumulus:~# net add interface swp32 breakout 4x25G

root@cumulus:~# systemctl restart switchd.service

How to Restore QSFP28 100G Ports to Default 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, 4*25GbE and 4*10GbE mode. The hardware required and the configuration commands are shown with FS S5850 48S2Q4C and N8500-32C, besides, the way to get back to the default mode of the switch port is of equal importance to the process.