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As stated in the documentation:
Aggregated Ethernet Interfaces
On QFX5100, QFX5120, EX4600, QFX10002 standalone switches, and on a QFX5100 Virtual Chassis and EX4600 Virtual Chassis, you can configure a mixed rate of link speeds for the aggregated Ethernet bundle. Only link speeds of 40G and 10G are supported. Load balancing will not work if you configure link speeds that are not supported.
That means that on qfx 5120 even if I try will not be possible to mix a 100g interface speed with a 40 g interface speed together? Should be enough to use the 100g for traffic and the 40g for standby if possible.