Port types and cages: what fits where

A faceplate mixes RJ45 jacks, SFP-family cages, QSFP-family cages and sometimes a combo port or a module slot. Which transceiver fits which cage is mostly mechanical backward compatibility within a family; whether it then works depends on the SerDes behind the cage and the NOS policy. This page is the fit-and-function matrix, with the adapters, combo ports and port-group rules that catch people out.

Cage families

Fit is necessary, not sufficient: the platform's supported-optics list decides the rest (Compatibility matrices).

RJ45 ports vs copper modules

Built-in RJ45 ports have a PHY on the board and negotiate 10/100/1000 (or 2.5/5/10G on multi-gig ports). A copper SFP (1000BASE-T, 10GBASE-T) puts that PHY inside the module; it draws more power than optics, may lack DDM, and 10GBASE-T SFP+ modules often exceed the cage's 1.5 W class (Copper Ethernet & DAC).

Combo ports

A combo port pairs one RJ45 and one SFP cage on a single switch port: only one is active. Rules vary — some platforms prefer the SFP when a module is present, others need media-type configuration. Symptom: an optic inserted into a combo port does nothing because the RJ45 side is active.

Adapters

An adapter presents the inner module's identity to the host; the host must accept both the adapter (if it has an EEPROM) and the module.

Breakout ports

A 4-lane or 8-lane cage can be configured as several logical ports when the optics are parallel (SR4, DR4, PSM4, CR4 DAC) and the ASIC/PHY mapping permits. Common constraints:

• only certain ports (often the first or last N, or every other port) can break out;
• breakout may disable the adjacent port;
• child ports inherit or require a FEC mode; all children run the same speed;
• CMIS modules must advertise the breakout application (CMIS).

Cabling side: Breakout & MPO cabling; configuration syntax: Port configuration recipes.

Speed groups and mixed-speed rules

Read the platform's hardware installation guide once; it saves hours of "the module is fine, why won't it link" (Speed & rate).

Uplink modules and expansion slots

Campus switches often take a removable uplink module (4 × SFP+, 2 × QSFP28…) with its own PHY/retimer and its own supported-optics list; stackable switches carry stacking ports that accept only stacking cables or specific DACs (Ports & uplinks).

Fit-and-function checklist

1. Cage family and generation → which modules fit and which speeds the SerDes can do.
2. Port-group rules → whether the neighbours constrain the speed.
3. Breakout capability of that specific port.
4. Power class of the cage vs the module.
5. NOS policy → supported list or override (How each NOS validates a module).
6. Module identity consistent with all of the above (Check transceiver).

In CodingBox

The identity bytes tell you what a module is; the cage and platform rules above tell you where it can go. CodingBox reads the former — form factor, rate codes, power class, connector — so the fit-and-function check can be done at the bench before the module travels to the rack (Check transceiver).

The same cages on server adapters, with PCIe bandwidth as the extra constraint: NIC ports, speeds and PCIe; what a 1G cage carries electrically and why copper SFPs care: SGMII & serial gigabit.

Interface names for these cages in every NOS, breakout child notation and LED codes: Port naming & LEDs; ports consumed by stacking and MLAG peer links: Stacking & MLAG.