Overview

Understanding the compatibility between QSFP28 100G transceivers and QSFP+ 40G switch slots is a critical pre-sales and engineering question for network architects. While the physical form factor is identical, the electrical signaling is fundamentally different. This FAQ addresses the most common B2B queries, covering compatibility rules, throughput, configuration steps, and troubleshooting to ensure successful deployment in enterprise and data center environments .

Frequently Asked Questions

Q1: Is the QSFP28 100G transceiver backward compatible with a QSFP+ 40G switch slot?

No, a QSFP28 100G transceiver is not compatible with a QSFP+ 40G port. While both modules share the same physical QSFP form factor and can be plugged into the same cage, a QSFP+ host switch ASIC lacks the 25 Gbps NRZ electrical signaling lanes required by a QSFP28 module . Inserting a QSFP28 into a 40G-only port typically results in a link failure or DDM errors .

Q2: Can I use an existing QSFP+ 40G transceiver in a new QSFP28 100G switch port?

Yes, QSFP28 ports are generally backward compatible with QSFP+ 40G modules. When you insert a 40G QSFP+ transceiver into a QSFP28 port, the link will typically auto-negotiate down to 40 Gbps . This allows for phased network upgrades, enabling you to reuse legacy 40G optics on new 100G-capable switches .

Q3: What is the maximum throughput when using a QSFP+ module in a QSFP28 port?

The maximum throughput is 40 Gbps. The link will operate at the native speed of the older QSFP+ module . The QSFP28 port will electrically downshift its 4x25 Gbps lanes to 4x10 Gbps to match the QSFP+ standard .

Q4: What are the key technical differences between QSFP+ and QSFP28 transceivers?

The primary differences are lane speed and total bandwidth. QSFP+ uses 4 electrical lanes at 10 Gbps each for a total of 40 Gbps (XLPPI interface), while QSFP28 uses 4 lanes at 25 Gbps each for a total of 100 Gbps (CAUI-4 interface) . QSFP28 modules also typically consume more power (3.5W – 5.5W) compared to QSFP+ (1.5W – 3.5W) .

Q5: How do I configure a QSFP28 port on a switch to work with a 40G QSFP+ transceiver?

In most modern switches, configuration is automatic via auto-negotiation. For instance, on Cisco Nexus 9000 or Aruba 6300 series switches, the port speed is automatically set to 40G when a QSFP+ module is detected . If auto-negotiation is not supported, you must manually set the port speed to 40G. For example, on FS S5850 switches, the command is speed 40G in interface configuration mode .

Q6: What cabling and breakout options are available for QSFP28 ports beyond 100G?

QSFP28 ports can operate in 4x25G or 4x10G breakout modes. To break out into 25G SFP28 links, you can use a 100G QSFP28 to 4x25G SFP28 breakout DAC or AOC cable . For 10G SFP+ connectivity, the port can be downshifted to 40G mode and then split into 4x10G using a QSFP+ to 4xSFP+ breakout cable .

Q7: How do I troubleshoot a link failure when inserting a QSFP28 module into a switch port?

First, verify that the switch port is QSFP28-capable and supports 100G. If the port is a QSFP+ 40G-only slot, the module will not work . Check the switch datasheet or use a CLI command like show system to confirm supported speeds . Also, verify that the port speed is correctly configured via CLI and that the switch firmware supports the specific transceiver .

Q8: Are 100G QSFP28 DAC cables compatible with a 40G QSFP+ port?

No, a 100G QSFP28 DAC cable is not directly compatible with a 40G QSFP+ port. The DAC cable uses 25 Gbps electrical lanes, which the 40G switch ASIC cannot support . For a 40G connection, you must use a matching 40G QSFP+ DAC cable .