Many network newbies may be confused by the combo port since the combo interfaces look similar to 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? Here, we will make a comprehensive introduction of the combo port and answer all the questions.
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 layers and share the same switch fabric and port number. 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.
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 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
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. 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. For 3900-24F4S, you can plug a CAT5/6 cable into the device directly without having to purchase a separate GLC-T copper SFP.
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.