When 400G is needed
400G is needed where a link must carry very high traffic: between data centre switches, between data centre sites, or between a data centre and its users, where 100G links are saturated and where aggregating more 100G links costs more than a single 400G link. It is not needed where the equipment is 100G or slower, because a 400G module cannot make a 100G port faster, and it is not needed where the traffic is low, because the module's capacity would sit unused.
What to match
Match the form factor to the equipment's slot: a QSFP-DD module fits a QSFP-DD slot, and a module for another form factor cannot be adapted without an adapter that adds loss and failure points. Match the reach to the link's distance, with margin, and the fiber type to the installed fiber, because a 400G module's reach differs by fiber type. Match the connector to the installed connectors and the equipment's ports. Confirm all four — form factor, reach, fiber type, connector — for the exact equipment, in the inquiry, before ordering.
The representative model
The QDD-400G-SR8 representative page, in the 400G category, carries a full specification table from its official product page, and its own trade terms for that page only. A buyer who needs 400G should start from that page, check its specification table, and confirm the form factor, the reach, the fiber type, and the connector for their exact equipment, before ordering.
What to confirm before ordering
Name the model, the equipment it must serve, the quantity, and the market. Ask for the specification for the exact model, the minimum order quantity, the lead time with its meaning, the warranty terms, the payment and shipping terms, and the certificates that apply to your market, by name, with documents attached.
Before payment, confirm the minimum order quantity, lead time, payment, shipping and warranty terms for your order. Certificates are available upon request.