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What are SFP ports on a Gigabit switch and what are they used for?

SFP ports enable Gigabit switches to connect to a wide variety of fiber and Ethernet cables in order to extend switching functionality throughout the network.

Small form-factor pluggable, or SFP, devices are hot-swappable interfaces used primarily in network and storage switches. The SFP ports on a switch and SFP modules enable the switch to connect to fiber and Ethernet cables of different types and speeds.

The Small Form Factor Committee, a group consisting of major equipment vendors, created the SFP specification and first published it in 2001. SFP modules built to the initial specifications support data rates of up to 1 Gb. The specification has since been updated to support increased data rates.

Almost all enterprise-class switches include two or more SFP ports, enabling them to become part of a ring- or star-based network topology spread among different buildings, floors or areas, connected via fiber optic cabling.

SFP media support

SFP module types support both single and multimode fiber and Cat5, Cat6, Cat6a and Cat7 twisted pair copper. SFP modules designed for use with fiber support a variety of wavelengths and maximum distances up to 2 km with multimode and 40 km with single mode.

Connections using Cat6a are limited to approximately 100 m, but cross-talk between adjacent cables can limit actual maximum distance. Cat7 cables include additional shielding to reduce cross-talk, but they can be difficult to install due to increased cable thickness.

The enhanced SFP (SFP+) specification was released in 2006 with a further update in 2011. SFP+ modules support rates up to 16 Gbps. Like the earlier SFP model, they support both fiber and copper, but copper interconnects are limited to 10 m at 10 Gbps. The SFP+ specification supports single-mode fiber interconnects to 40 km, but some vendors support up to 80 km distances.

SFP+ modules are specified for Fibre Channel (FC) at up to 8 Gbps, but some vendors support up to 16 Gbps rates. SFP+ also supports WAN connections based on Optical Transport Network specification OTU2.

Some SFP modules, such as GLC-BX-D and GLC-BX-U, support digital optical monitoring, which lets end users monitor the performance of SFPs in real time, tracking metrics, such as temperature, optical output power, optical input power, transceiver supply voltage and laser bias current.

SFP ports vs. RJ45 ports

While SFP and SFP+ modules are relatively inexpensive, 1 Gb and 10 Gb connections are more expensive than RJ45 connections. RJ45 uses copper cables, which are generally less expensive than fiber. Also, RJ45 ports are typically built into switches so customers don't need to purchase additional parts.

The decision of where to use an RJ45 port or an SFP port on a switch depends on the distance between the connection endpoints.

The decision of where to use an RJ45 port or an SFP port on a switch depends on the distance between the connection endpoints. If the endpoints are close enough to use copper, then using RJ45 is less expensive. If the distance is great enough to require fiber, then teams should use SFP ports. Fiber is also a better choice where cables must run side by side for a distance that makes cross-talk between copper cables an issue.

Combo SFP ports on a switch offer support for both SFP and RJ45 ports using a single interface. Users can't use both ports simultaneously, however, as only one type will work at a time. These combo ports enable IT teams to choose a connection type that best matches application, network, cost or distance requirements.

SFP types and updates

Vendors have continued to update SFP specifications to support higher rates and additional interconnect types. SFP28 supports 25 Gbps over both multimode and single-mode fiber. BiDi SFPs support bidirectional communication over a single fiber. While other SFP types require two fibers -- one to send and one to receive -- BiDi SFPs use different wavelengths with one wavelength for sending data and the other for receiving.

Compact SFPs support two bidirectional links using the same interface module -- two fibers are used with each carrying bidirectional traffic. Quad SFP (QSFP) modules support four fibers carrying Ethernet, FC or InfiniBand. Most available QSFP modules can support up to 200 Gbps. Both octal SFP and QSPF double density have recently become available and extend data rates to 400 Gbps.

Currently, 400 Gbps is the highest rate available in commercially available products, although work is underway to develop standards and products that support 800 Gbps. SFP technology has demonstrated the ability to evolve as demands increase and will likely continue to evolve to support higher rates.

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