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Can 10Gb Switch Port Link to Gigabit Switch Port?

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Posted on December 25, 2019
September 22, 2020
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With the tendency for a higher-speed network, 10Gb switch has gained more and more popularity among home and enterprise networks. However, the issue of legacy SFP to SFP+ compatibility always confuses many network switch users. Will 1Gb SFP optics work with 10Gb SFP+ ports on a 10Gb switch? Or will 10Gb SFP+ run at 1Gb to link gigabit switch? And whether 10Gb optics on a switch can auto-negotiate to 1Gb when the other end is 1Gb? All these related questions originate from the link between 10Gb SFP+ slots on 10Gb switch and 1Gb SFP interfaces on the Gigabit switch. Thus we will reveal the mask of SFP to SFP+ compatibility from this point of view.

What Is SFP Port on Gigabit Switch?

Ethernet switch, known as LAN (Local Area Network) switch is the most common form of network switch, of which fixed-configuration switches with different speeds are the most used, including Fast Ethernet switch (100 Mbps), Gigabit switch (1000 Mbps), 10Gb switch (10000 Mbps), as well as 25G/40G/100G switches. As the sizes and data rates of Ethernet network switches vary, the switch port types also do. SFP (small form-factor pluggable) port on network switch is a compact, hot-pluggable network interface. Typical speeds were 1 Gbit/s for Ethernet SFPs and up to 4 Gbit/s for Fiber Channel SFP modules. SFP port (electrical port and optical port) enables a gigabit switch to achieve fiber uplink over longer distances or short-range copper uplinks by inserting the corresponding SFP module (fiber SFP or copper SFP).

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Figure 1: SFP Port and Uplink SFP+ Port on Gigabit Switch.

What Is SFP+ Port on 10Gb Switch?

SFP+ and SFP ports are virtually identical in size and appearance. The primary difference is that SFP+ is an updated version that supports higher speeds up to 10Gbps. And there are mainly two types of 10Gb switch, including 10G fiber switch and 10GBASE-T switch both could have higher-bandwidth uplink ports (40G/100G) to connect with other higher-speed switches in the topology.

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Figure 2: SFP+ Port and Uplink QSFP+ Port on 10Gb Switch.

In Most Situations, 1G SFP Optics Can Be Used on 10G SFP+ Port

Will 1G SFP transceivers work with 10G SFP+ ports? The answer is "Yes" in most cases. SFP+ ports often accept SFP optics but at a reduced speed of 1Gb. And sometimes this port will lock the speed at 1Gb until you reload the switch or do some fancy set of commands. Besides, the SFP+ port usually can never support speed under 1G. That is to say, we can’t plug the 100BASE SFP in the SFP+ port. In fact, for this question, it may depend greatly on the switch models – sometimes SFPs are supported in SFP+ ports, and sometimes not. There are many vendors providing 10Gb switches that can take both a 10G SFP+ transceiver and a 1G SFP in the 10G SFP+ slot, but not at the same time. This option is supported by dual-rate operation. So before plugging a SFP transceiver into the SFP+ port on your 10Gb switch, one must consult the vendor to make sure the 10Gb switch port supports dual rate. When connecting a 10Gb switch port to a gigabit switch port, just install 1Gb SFP optics on the 10Gb switch SFP+ port and on the gigabit switch 1Gb SFP port respectively, then connect the 10Gb switch and the gigabit switch with a corresponding fiber patch cable or Ethernet copper cable.

In All Cases, 10Gb SFP+ Optics Cannot Be Used on 1G SFP Port

Will 10Gb SFP+ running at 1Gb? The answer is definitely “No”. SFP optics do work in SFP+ slots in most cases, but 10Gb SFP+ optics can never work in SFP slots on gigabit switch because SFP+ does not support speeds less than 1Gbps. As we know, once a module is installed, the speed of the port is decided. Most SFP+ slots are backward compatible with SFP modules to run at 1G speed. However, the SFP slots on Gigabit switch cannot support the 10G speed required by SFP+ modules. For instance, most 10Gb switches support 10G SFP+ and 1G SFP optics on SFP+ ports. But some models are SFP+ only. One needs to double-check the compatibility of this switch with the vendor rep.

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Figure 3: FS 10Gb Switch SFP+ Port Links to Gigabit Switch SFP Port.

SFP+ Optics Cannot Auto-negotiate Down to 1G with Gigabit Switch

Unlike copper SFP modules supporting 10/100/1000 auto-sensing, fiber SFP and SFP+ optics do not support auto-negotiation as this technology is based on electrical pulses, not optical pulses. Actually, auto-negotiation is used in twisted pair by which two connected devices choose the highest performance transmission mode they both support after sharing their capabilities. It only works when it’s running on both sides of the link. Thus 10Gb SFP+ optics on 10Gb switch can not auto-negotiate down to 1Gb if the other end is gigabit switch. In fact, most SFP and SFP+ transceivers only run at its rated speed and the transceivers at both ends of the cable should at the same speed. For example, if a 10Gb SFP+ module is plugged into the 10Gb switch port, it will only run at 10Gb. In this case, if you link it to the gigabit switch port, it will not work. When plugging a 1Gb SFP module in the 10G SFP+ port, the 10Gb switch will only run at 1Gb. Thus you can link it to the Gigabit switch.

Conclusion

For the issue of SFP to SFP+ compatibility, a simple answer is that most SFP+ can take SFP but not vice versa. Thus 10G SFP+ optics on 10Gb switch port is possible to link to 1G gigabit switch port when 1Gb SFP modules are plugged into each port. To make sure the smooth network transmission, you need to consult the switch vendor to ensure the connectors on both ends are the same and no manufacturer compatibility issue exists. If needed, FS is pleased to help you at any time.

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