"When I saw an Ethernet switch for the first time, I was confused by the different ports on it, especially the combo ports." Many network newbies may be troubled by the combo port since the appearance of the combo interfaces is similar to that of common Ethernet ports. So what is network combo port? How to differentiate the combo ports from Ethernet ports on an Ethernet switch? How to use combo SFP port? A series of questions are coming all the time. Here, we will make a comprehensive introduction to the combo port and explain all the questions for you.
A combo port, also known as optoelectronic multiplexing interface, is the photoelectric composite port (panel) 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 and share the same switch fabric and port number. What separates these ports from being individual is that they're tied to the same switch fabric. However, the two different ports can not be used simultaneously. That is to say, when RJ45 port is activated, the SFP port is automatically disabled and vice versa.
Usually, the RJ45 port is connected by the twisted pairs while the SFP port needs copper or optical modules and patch cables. The obvious merit of combo interface is that you still have more flexibility in terms of choosing your network connectivity type without taking up too much space. Take FS S3900-24F4S switch as an example, it has 20 1000BASE-X SFP ports and 4 combo ports. Therefore, the switch can be configured to have 24 SFP ports or 20 SFP ports and 4 RJ45 ports.
Figure 1: Combo Ports on S3900-24F4S Switch
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 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 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.
The combo interface has an electrical port and an optical port. The electrical port and its corresponding optical port are logically multiplexed. The RJ45 Ethernet ports are usually used for connecting switches closer than 100m apart, while SFP ports are for longer fiber connections according to the fiber types. You can choose which one to use according to your actual network requirements.
SFP port (small form-factor hot-pluggable interface)，also called mini-GBIC port, is designed for Gigabit data transmission. The SFP ports can support both copper and optical links. Regardless of the optical port or electrical port link, the only difference is the physical layer. When copper SFP module is inserted in the SFP port, Ethernet copper cables (Cat5/Cat5e/Cat6) are usually deployed for data transmission. And fiber SFP module and LC patch cable are needed for fiber optic transmission over longer distance.
1. You can use either optical ports or electrical ports of the combo ports, but not both at the same time.
2. The SFP slot takes priority when you connect both twisted-pair port and SFP slot of a combo interface pair to network devices.
3. The SFP slot becomes active when the SFP transceiver establishes a link to a network device.
4. The twisted-pair port and SFP slot of a combo port pair share the same settings, such as VLAN (Virtual Local Area Network) assignments, access control lists, and spanning tree.
For combo ports on Ethernet switch, what needs to be highlighted again is 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 a combo port based on the interface identifier on the switch panel. 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.