Juniper Port Checker – Validate Port Speed Mappings Before You Deploy

If you work with Juniper hardware and have never used the Juniper Port Checker, you are missing out on a really useful tool. It is part of the Juniper Pathfinder suite and it gives you a visual representation of the front panel of a device and lets you configure port speeds to validate that your planned configuration is actually supported by the hardware. No more digging through data sheets trying to figure out what happens to adjacent ports when you change a speed.

Let me walk through how it works using the QFX5120-48Y as the example, since that is a switch I have been spending a lot of time with lately.

What is the Port Checker?

Head over to apps.juniper.net/port-checker and you will see a catalog of Juniper devices organized by product family. Type the model into the filter box and click your device. That is it – you land on a visual front panel view with color-coded ports showing the supported speed ranges at a glance.

There are three tabs – Port Speeds, Channelization, and Port Groups. The Port Speeds tab is where most of the useful work happens. You can click any port on the diagram to set a speed, and the tool immediately validates the entire configuration and tells you if it is valid or not.

The QFX5120-48Y Default Layout

When you load the QFX5120-48Y you see two groups of ports:

• Ports 0–47 (48 ports) – yellow/green, labeled 1/10/25GbE. These are the SFP28 front ports.
• Ports 48–55 (8 ports) – blue, labeled 10/25/40/100GbE. These are the QSFP28 uplink ports.

The default state shows all 48 front ports capable of 1G, 10G, or 25G, and the 8 uplink ports capable of 10G, 25G, 40G, or 100G. Configuration Status shows VALID in green.

What Happens When You Change Port Speeds

This is where it gets interesting. The QFX5120-48Y uses a Broadcom Trident 3 ASIC, and the 48 front ports are organized into groups. When you change a port speed it can affect the neighboring ports in the same port group because they share ASIC resources. The Port Checker shows you exactly what that impact is.

10G (Default)

10G is what you get out of the box. All 48 front ports default to 10G and you still get full port density with each port operating independently. Interfaces show up as et- in JunOS and you need an SFP+ optic. It is a common starting point if you are connecting to older ToR switches or servers that only have 10G NICs.

25G

To get 25G you click a port in the diagram and select 25G from the dropdown. The layout stays the same – all 48 ports still show individually and the configuration stays green. Interfaces come up as et- in JunOS just like at 10G, but you need an SFP28 optic. This is the native speed the switch was designed for and what you want for modern server connectivity. The ASIC handles it cleanly and you do not lose any ports in the group when you make the switch.

1G

This is where things get more interesting. Set a port to 1G and the tool still shows it as valid, but there is a caveat you need to know – the QFX5120-48Y does not support 1G natively on all ports simultaneously without a few restrictions. In JunOS when you configure a port at 1G the interface comes up as xe- rather than et-, and you will see a warning triangle on the port in the diagram. You also need a 1G SFP optic (not SFP+/SFP28) or a DAC that auto-negotiates down.

The key constraint the Port Checker surfaces here is that within a port group, you cannot mix 1G and 25G on the same group of ports in some configurations. If you are planning to run a mix of 1G servers and 25G servers off the same switch, the Port Checker will tell you immediately whether your port assignment plan is valid or whether you need to rethink which physical ports you use.

The Port Groups Tab

While you are in there, click over to the Port Groups tab. This is really useful because it shows you exactly which physical ports share an ASIC resource group. On the QFX5120-48Y the 48 front ports are divided into groups of ports that are managed together by the ASIC. Knowing this grouping is essential when you are planning a mixed-speed deployment – you want all your 1G ports in the same group and all your 25G ports in another, rather than splitting them.

Why This Tool is Actually Useful

Before this tool existed you would have to cross-reference the hardware guide, the release notes, and maybe a JTAC case to figure out whether your planned port configuration was going to work. I have run into situations in the field where someone had a mixed 1G/25G plan and did not realize until the switch arrived that their port assignments were going to cause problems.

The Port Checker is not just for QFX switches either – it covers MX routers, ACX, PTX, and more. If you are doing any kind of pre-deployment planning or troubleshooting a port speed issue, it is worth bookmarking. You can find it at apps.juniper.net/port-checker.