What Does Combo Port Mean for Ethernet Switch?

Updated on Sep 11, 2021 by

The switch combo ports may have confused many network newbies since the combo interfaces look similar to Ethernet ports. So what is network switch combo port? How to differentiate the combo ports from Ethernet ports on an Ethernet switch? How to use combo SFP port? We will make a comprehensive introduction of the combo port and answer all the questions.

What Is Combo Port?

A combo port, also known as an optoelectronic multiplexing interface, is a photoelectric composite port with two kinds of Ethernet interfaces (RJ45 port and SFP port) on an Ethernet switch. In other words, it is a compound port that can support two different physical layers and share the same switch fabric and port number. However, the two different ports cannot be used simultaneously. When the RJ45 port is activated, the SFP port is automatically disabled, and vice versa.

Typically, the RJ45 port is connected by twisted pairs, while the SFP port requires copper or optical modules and patch cables. Combo ports provide flexibility and versatility in network deployments by allowing the use of different media types on the same port. This adaptability accommodates various networking scenarios and reduces the need for separate dedicated ports for each media type, saving space and costs while simplifying network management.

Combo ports are commonly found in enterprise-grade switches and are useful in environments with a mix of devices having different connectivity options. They are particularly beneficial during transitions from copper to fiber infrastructure, enabling a smooth migration without a complete overhaul of the network infrastructure.

Introduction to FS Combo Port Switch

The FS S3900-24F4S switch is an excellent example of a device that leverages the benefits of combo ports. It features four combo ports, facilitate the use of different connectivity, making it more flexible to configure the switch. It has a console port, 4 x 1G RJ45/SFP Combo, 20x 1Gb SFP and 4x 10Gb SFP+ Uplinks. It offers up to 128Gbps switching capacity to simultaneously process traffic on all ports at line rate without any packet loss.

Figure 1: Combo Ports on S3900-24F4S Switch

To identify a combo port, there is the interface identifier on the switch panel. If two ports have the same ID but should connect to different transmission media, the two ports are multiplexed as a combo port. You can also run the display interface command to display the combo port. Take FS S3900-24F4S combo port switch as an example, it contains 4 combo ports (4 copper and 4 sfp ports but only one type can be used at a time). Just enter "show interface brief" in the CLI. You can see the status of each port. Port 21 - 24 are the combo ports. S3900-24F4S switch is handy as there is no need to specify the "media-type" of the combo interface to tell the device to use either the SFP or the RJ-45, just plug 1G SFP transceivers or CAT5/6 cables (but not both at the same time) into these ports.

Figure 2: Combo port of S3900-24F4S gigabit switch shown in CLI

Types of Combo Ports

There are two kinds of combo interface types, that is single combo port and dual combo port designed for simple management of the Ethernet network.

Single Combo Port

Single combo port is the two Ethernet ports on the device panel correspond to one port view. The user performs the state switching operation of two ports in the same port view. A single combo interface can be a Layer 2 or Layer 3 Ethernet interface.

Dual Combo Port

Dual combo port is the two Ethernet interfaces on the device panel correspond to the two different port views. The user can switch the status of the two ports on the optical or electrical port view. However, the dual combo interface can only be a Layer 2 Ethernet interface.

Comparison of Combo Ports, SFP Ports, and RJ45 Ports

Combo Ports

Combo ports feature both electrical (RJ45) and optical (SFP) connections that are logically multiplexed, meaning only one type can be used at a time. The electrical port, typically an RJ45 connector, is used for connections within 100 meters, while the optical port, usually an SFP connector, can be used for longer-distance links depending on the type of fiber used. The key advantage of Combo ports lies in their flexibility, allowing users to choose the type of network connection based on their needs without taking up additional space. For instance, on the switch 3900-24F4S, you can directly plug in a CAT5/6 cable without requiring a separate GLC-T copper SFP module.

SFP Ports

An SFP (Small Form-factor Pluggable) port, also known as mini-GBIC port, is primarily used for Gigabit data transmission and support both copper and fiber connections. The difference between using an electrical or optical port lies in the physical layer. When a copper SFP module is inserted, Ethernet copper cables (Cat5/Cat5e/Cat6) are typically used for data transmission. In contrast, fiber SFP modules require LC jumpers for long-distance optical fiber transmission.

RJ45 Ports

RJ45 ports are eight-pin connectors widely used for Local Area Network (LAN) connections, especially Ethernet. They are mainly used for short-range network connections, usually within 100 meters, with common cable types being Cat5, Cat5e, and Cat6. These cables can support 10Mb, 100Mb, and 1Gb Ethernet data transmissions. The primary advantage of RJ45 interfaces is their simplicity in installation and use, requiring no specialized fiber infrastructure or related equipment, making them more suitable for short-range LAN connections.

Rules for Using Combo Ports

Combo ports offer flexibility by allowing the use of either optical or electrical connections within a single port configuration. This adaptability can simplify network design and enhance connectivity options. Here are some important rules and considerations for using combo ports effectively:

1. Single Port Usage: You can use either the optical port or the electrical port of the combo port switch, but not both simultaneously. This means that when one type of port is active, the other is automatically disabled. This design prevents conflicts and ensures seamless network operation.

2. Priority of SFP Slot: When both the twisted-pair port and the SFP slot of a combo interface pair are connected to network devices, the SFP slot takes priority. This is because the SFP slot, which supports fiber optic connections, generally offers higher performance and longer distance capabilities compared to twisted-pair connections. As a result, when a connection is established through the SFP slot, it will be the active link.

3. Activation of SFP Slot: The SFP slot becomes active when the SFP transceiver establishes a link with a network device. This means that if you insert an SFP transceiver into the slot and it successfully connects to another device, the switch will automatically activate the SFP slot and disable the corresponding twisted-pair port.

4. Shared Settings: The twisted-pair port and SFP slot of a combo port pair share the same configurations, including VLAN (Virtual Local Area Network) assignments, access control lists (ACLs), and spanning tree settings. This shared configuration ensures that whichever port is active, it adheres to the same network policies and security protocols, maintaining consistency across the network.


For combo ports on Ethernet switch, it is highlighted that optical or electrical ports can't be used at the same time. Generally, the combo ports are labeled by vendors. If you're not sure, you can identify it based on the interface identifier on the combo port switch. If two ports have the same ID but connect to different transmission media, the two ports are multiplexed as a combo port. From above discussion, hope you have a general understanding of the network combo port.

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