The following article describes how to channelize interfaces in Juniper Networks QFX5120-32C Series switches. The switch supports the following port speeds:

40-Gbps and 100-Gbps (non-channelized mode).

10-Gbps, 25-Gbps, and 50-Gbps (channelized mode).

Using 100GbE QSFP28 ports 0-30 can be channelized into four 25GbE SFP28 ports and this works using breakout cables or regular non-breakout cables as well.
Also, 40GbE QSFP+ ports 0-30 can be channelized into four 10GbE SFP+ ports, increasing the total number of 25GbE per switch to 124 and the total number of 10GbE ports per switch to 126 (including SFP+ ports 32 and 33). Port 31 does not support 4x10G or 4x25G channelization options. Only 100GbE, 40GbE, and 2x50GbE channelization options are supported on port 31.

Without this configuration the channelized interfaces might not come up.

To channelize the ports, manually configure the port channel speed using the set chassis fpc slot-number pic pic-slot port port-number channel-speed speed command, where the speed can be set to 10G, or 25G, or 50G. For example:

set chassis fpc 0 pic 0 port 10 channel-speed 10g

2024-01-15: KB modified, added PIC keyword. 
2025-10-15: KB modified, added the the confirmation that 4x25G channelized interfaces might work as well with regular non-breakout cables after lab test and also customer confirmation.