{"id":101716,"date":"2026-10-20T10:41:36","date_gmt":"2026-10-20T10:41:36","guid":{"rendered":"https:\/\/optics.carritech.com\/?p=101716"},"modified":"2026-10-20T10:52:01","modified_gmt":"2026-10-20T10:52:01","slug":"cfp-cfp2-cfp4","status":"publish","type":"post","link":"https:\/\/optics.carritech.com\/pt\/cfp-cfp2-cfp4\/","title":{"rendered":"CFP, CFP2 and CFP4: the 100G optics still holding up production networks"},"content":{"rendered":"

Nobody designs a new network around CFP. Nobody has for a decade. And yet CFP, CFP2 and CFP4 modules are still carrying production traffic in core routers, transport platforms, firewalls and aggregation switches all over Europe, because the line cards they plug into are expensive, they work, and nothing has given anybody a sufficiently good reason to rip them out.<\/p>\n\n\n\n

That makes CFP an awkward thing to own. The modules are getting harder to buy from the original vendors, the lead times are getting worse, and the engineers who knew the quirks have mostly moved on to QSFP-DD. This guide covers what the three form factors actually are, how they differ, the one CFP variant that still has a future, what your options are if you own CFP line cards, and which modules fit what.<\/p>\n\n\n\n

Where CFP came from<\/h2>\n\n\n\n

100 Gigabit Ethernet was standardised in IEEE 802.3ba<\/a>, approved in June 2010. At that point nobody could build a 25G serial lane economically, so 100G was constructed as ten lanes of 10G<\/strong> \u2014 ten lanes on the electrical side, and on multimode, ten lanes of fibre each way.<\/p>\n\n\n\n

Ten electrical lanes plus the optics and the thermal mass to run them needs a large package, and CFP \u2014 "C form-factor pluggable", the C from the Roman hundred \u2014 is what the industry produced. It is a big module by any standard: roughly 145 mm deep, 82 mm wide, 13.6 mm tall. You can hold one and feel the generation it came from.<\/p>\n\n\n\n

Then 25G serial lanes arrived, 100G was rebuilt as four lanes of 25G, and each shrink of the electrical interface produced a smaller package: CFP2, then CFP4, then QSFP28 \u2014 which is where the market stopped, because QSFP28 was small enough to put 32 of them on a 1U switch. CFP8 exists for 400G and is a historical footnote; the 400G market went to QSFP-DD and OSFP<\/a> instead.<\/p>\n\n\n\n

The family, compared<\/h2>\n\n\n\n
\n\n
<\/th>PCP<\/th>CFP2<\/th>CFP4<\/th>QSFP28<\/th><\/tr><\/thead>\n
Approx. size (D × W × H)<\/td>145 × 82 × 13.6 mm<\/td>107.5 × 41.5 × 12.4 mm<\/td>92 × 21.5 × 9.5 mm<\/td>72 × 18.35 × 8.5 mm<\/td><\/tr>\n
Interface el\u00e9ctrica<\/td>10 × 10G (CAUI-10)<\/td>10 × 10G or 4 × 25G<\/td>4 × 25G (CAUI-4)<\/td>4 × 25G<\/td><\/tr>\n
Management<\/td>MDIO<\/td>MDIO<\/td>MDIO<\/td>I²C<\/td><\/tr>\n
Typical max power<\/td>up to ~24 W<\/td>up to ~12 W<\/td>up to ~6 W<\/td>~3.5 W<\/td><\/tr>\n
Ports per 1U, realistically<\/td>4–8<\/td>8–16<\/td>~24<\/td>32–36<\/td><\/tr>\n
Status<\/td>Legacy, support only<\/td>Legacy — except coherent<\/td>Legacy, never widely adopted<\/td>Current<\/td><\/tr>\n<\/tbody>\n<\/table><\/figure>\n\n\n\n

Carritech’s own CFP-family modules bear that progression out: the CT-CFP2-LR4<\/a> is specified under 9 W and the CT-CFP4-LR4<\/a> under 6 W for the same 100GBASE-LR4 interface over the same fibre to the same 10 km.<\/p>\n\n\n\n

MDIO: the difference that actually bites<\/h2>\n\n\n\n

Every CFP-family module is managed over MDIO<\/strong> \u2014 the IEEE 802.3 Clause 45 management interface, a clocked two-wire bus with a 16-bit register model inherited from Ethernet PHY management. Every SFP, QSFP and QSFP-DD module is managed over I²C<\/strong>, with the byte-oriented memory map defined in the SFF specifications maintained through SNIA<\/a> and, on the newest form factors, the OIF’s Common Management Interface Specification<\/a>.<\/p>\n\n\n\n

Three consequences worth knowing:<\/p>\n\n\n\n