Yes, QSFP-DD (Quad Small Form-factor Pluggable Double Density) is designed to be backward compatible with QSFP28 (Quad Small Form-factor Pluggable 28). QSFP-DD is an enhanced version of the QSFP form factor that supports double the number of electrical lanes, allowing for higher data rates. 

Key points regarding compatibility:

1. Mechanical Compatibility:

• QSFP-DD retains the same basic form factor and size as QSFP28, ensuring mechanical compatibility. This means that a QSFP-DD module can physically fit into a QSFP28 port. 

2. Electrical Compatibility:

• QSFP-DD modules have additional electrical connectors, effectively doubling the number of lanes compared to QSFP28. Despite this increase in lanes, QSFP-DD modules are designed to be backward compatible with QSFP28 ports. This means that a QSFP-DD module can be inserted into a QSFP28 port, and the electrical interface will operate at the QSFP28 data rate.

3. Higher Data Rates:

• The primary advantage of QSFP-DD is its ability to support higher data rates by using additional lanes. While QSFP28 typically supports data rates up to 100 Gbps per channel, QSFP-DD can support data rates of 200 Gbps or 400 Gbps by utilizing double the number of lanes.

4. Compatibility Modes:

• Some networking devices may support both QSFP28 and QSFP-DD modules and provide compatibility modes. In a compatibility mode, a QSFP-DD port can operate at the QSFP28 data rate, allowing for mixed deployment of QSFP28 and QSFP-DD modules within the same network infrastructure.

When planning network upgrades or deployments, it's essential to check the compatibility of the networking equipment and ensure that the devices can support the specific QSFP modules you intend to use. The backward compatibility of QSFP-DD with QSFP28 provides flexibility for network operators to transition to higher data rates gradually.