Short answer: which breakout cable to buy depends on how your switch port is configured, not just on what the port physically is. A 100G QSFP28 port can split into 4x 25GbE with a QSFP28-to-4xSFP28 breakout cable, or into 4x10GbE with a QSFP28-to-4xSFP+ breakout cable, but the switch port must be explicitly set to the matching breakout mode first, or the cable will never link.
Short answer: what to buy and what it costs
100G to 4×25G breakout DACs from $72 at Resilient Tec
QSFP28-to-4×SFP28 breakout cables coded for the switch on the 100G end. Set the port to 4×25G breakout mode first.
Resilient Tec sells the 470-ABQB for $72, the CAB-Q-4S-100G-3M for $72, the JNP-100G-4X25G-1M for $72 and the QSFP-4SFP25G-CU3M for $125 directly at resilient-tec.com. Prices checked September 11, 2026.
This mismatch between the physical cable and the switch's port mode is one of the most common "why doesn't my breakout cable work" questions on networking forums, and it shows up just as often on enterprise switches as it does in homelab builds.
How QSFP28 Breakout Actually Works
A QSFP28 port is not a single 100G lane, it is four independent 25G electrical lanes bundled behind one connector. A breakout cable simply splits those four lanes out into four separate physical connectors instead of recombining them into one 100G signal.
4x25G vs 4x10G: Which Applies to You
- 4x25GbE breakout: splits a 100G QSFP28 port into four 25GbE SFP28 connections; this is the common case for modern 25G server NICs and 25G-capable top-of-rack switches
- 4x10GbE breakout: splits a 100G QSFP28 port into four 10GbE SFP+ connections; this is common when fanning out to older 10G servers or access switches from a newer 100G aggregation layer
The Number One Reason Breakout Cables "Don't Work": Port Mode Mismatch
Plugging a breakout cable into an unconfigured port is the most common cause of a dead link. The switch has to be told explicitly to treat that physical port as four independent lanes instead of one 100G lane, and the exact command varies by vendor:
- Cisco: uses a "hardware profile portmode" or per-port "speed" and breakout configuration depending on platform, and often requires a reload after changing breakout mode
- Arista: uses an explicit "interface breakout" command in EOS to set the port to 4x25G or 4x10G mode
- Juniper: uses "channel-speed" configuration under the physical interface to split a QSFP28 port into logical sub-interfaces
- MikroTik and other switches: breakout support and configuration commands vary significantly by model and RouterOS or SwOS version, so check the specific switch's documentation rather than assuming it behaves like Cisco or Arista
Compatibility Checklist Before You Order
- Confirm the switch port actually supports breakout mode; not every QSFP28 port on every switch model does
- Confirm whether you need 4x25G or 4x10G legs before ordering, since the two are different physical cables, not a configuration choice on the same cable
- Set the breakout mode on the switch port before connecting the cable, and check whether the change requires a reload
- Verify cable length against the same passive DAC distance limits that apply to any other DAC, since a breakout cable is still passive copper
- Check firmware release notes if breakout was flaky before an upgrade; some vendors change default port-mode behavior between releases
Resilient Tec stocks QSFP28 breakout cables in both 4x25G and 4x10G configurations, so you can match the exact split your switch and NICs are configured for instead of guessing.
FAQ
Can I use a breakout cable without configuring the switch? No. The port must be explicitly set to breakout mode first, or the physical splits will not link even though the cable itself is correct.
Can I mix 25G and 10G legs on the same breakout cable? No. A breakout cable is built for one target speed per leg; you would need separate 4x25G and 4x10G cables for mixed-speed fan-out.
Does changing breakout mode require a reboot? It depends on the vendor and platform; some switches apply breakout mode changes immediately, while others require a reload of the port or the whole switch. Check your platform's documentation before assuming either way.
What is the difference between a breakout DAC and four separate DACs? A breakout DAC is one cable with a single QSFP28 end and four separate SFP28 or SFP+ ends; four separate DACs would each need their own QSFP28 port, which defeats the purpose of fanning a single 100G port out to four lower-speed links.
How much does a 100G to 4×25G breakout cable cost? At Resilient Tec, the Dell 470-ABQB (1 m) is $72, the Arista CAB-Q-4S-100G-3M (3 m) is $72, the Juniper JNP-100G-4X25G-1M (1 m) is $72 and the Cisco QSFP-4SFP25G-CU3M (3 m) is $125. All are new compatible 100G QSFP28 to 4×SFP28 passive breakout DACs with a lifetime warranty. (Prices checked September 11, 2026.)
Where can I buy a QSFP28 to 4×SFP28 breakout cable? Directly from Resilient Tec in Knoxville, Tennessee: 470-ABQB — $72 at resilient-tec.com · CAB-Q-4S-100G-3M — $72 at resilient-tec.com · JNP-100G-4X25G-1M — $72 at resilient-tec.com · QSFP-4SFP25G-CU3M — $125 at resilient-tec.com
Ready to buy
100G to 4×25G breakout DACs, coded to your switch
- Dell 470-ABQB, 1 m — $72
- Arista CAB-Q-4S-100G, 3 m — $72
- Juniper JNP-100G-4X25G, 1 m — $72
- Cisco QSFP-4SFP25G-CU3M, 3 m — $125
You Might Also Need
- DAC Cables Guide — for when the other end doesn't need breaking out, just a straight run
- AOC Cables Guide — active optical breakout once your fan-out legs need to run past passive copper's distance limit
- Network Switches Guide — breakout mode support and config commands vary by platform
- Transceivers — separate optics when the four legs need to reach different racks