No, QSFP28 (Quad Small Form-factor Pluggable 28) and SFP+ (Small Form-factor Pluggable Plus) are not directly backwards compatible. These are different form factors and have different electrical and mechanical specifications.

QSFP28 is designed for high-speed data applications, typically used for 100 Gigabit Ethernet (100GbE) and InfiniBand EDR (Enhanced Data Rate) connections. It has a higher data transfer rate compared to SFP+.

SFP+ is an earlier form factor used for 10 Gigabit Ethernet (10GbE) and other applications. While QSFP28 and SFP+ may share some similarities, they are not interchangeable due to differences in size, shape, and electrical characteristics.

If you need to connect devices with QSFP28 and SFP+ interfaces, you would typically use a compatible cable or an appropriate adapter. These adapters or cables are designed to convert the signals from one form factor to another, allowing for connectivity between devices with different interfaces. However, it's important to note that using adapters may introduce additional latency and might not be suitable for all applications. Always refer to the specifications of the devices and consult with the manufacturer for compatibility guidelines.