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[Juniper Apstra] QFX5120 device interfaces not coming up if mixed port speeds are configured



Article ID: KB37520 KB Last Updated: 20 Sep 2021Version: 1.0

On QFX5120 device models, all SFP28 interfaces within the same "quad" (also known as "port group") must be configured with the same speed. If a mix of different speeds is used, the interfaces may not come up. This article describes how to use Juniper Apstra to ensure that the port and interface configurations are correctly configured on the device.


Interfaces may not come up when a mix of different speeds is used on ports that are within the same quad. There is no specific indicator of this problem, however, and configurations are applied as normal.


SFP28 ports are grouped into "quads," with each quad having four ports. For example ports 0/0/0, 0/0/1, 0/0/2, and 0/0/3 would be in the same quad, as would ports 0/0/4, 0/0/5, 0/0/6, and 0/0/7.

Port speeds are configured under the "chassis" configuration on the device. Only the first port in a quad can have the speed set, and this speed will apply to each port in that quad. For example, a typical configuration of setting the speed for ports 0/0/4 to 0/0/7 to 25G would look like this:

chassis {
   fpc 0 {
       pic 0 {
           port 4 {
               speed 25g;

When using an interface that is *not* the first port in a quad, Apstra will automatically ensure that the speed is configured on the first port of that quad. So for example, if in Apstra you have a QFX5120 device and are setting interface et-0/0/5 to 25G, the chassis configuration will look like above. 

The problem, however, occurs when multiple speeds are used within the same quad. This is a configuration that is accepted by Apstra and can be configured on the device, but it is not a valid use of the SFP28 ports and will not work.

This is because Apstra will not know which speed it needs to configure under the chassis configuration. How it works is that the logic iterates through each interface within the same quad that are active, and use the speed on the last one. Since the speed can only be configured on the first port of the quad, a different speed from what is expected will be configured on the port.


Having a mix of different speeds on the same quad is unsupported and MUST be corrected by ensuring that all speeds are set to the same value within each quad. If this is the case, the correct speed will be applied under the "chassis" configuration.

In Apstra, the speed on server facing ports can be changed by navigating to "Blueprint > Staged > Physical" and clicking the relevant device. Use the "Edit Links > Change link speeds" button to review currently configured speeds and change speeds such that they are the same on each quad.

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