This article explains about the feature "link-speed mixed" unsupported on EX4600 models on 21.4R3-S7 version.

Post upgrade on 21.4R3-S7, The mixed-rate speeds on aggregated ethernet interfaces will cause mismatch & ae interface to go down with trigger of following errors:

user@switch> show interfaces terse | match ae0 
ae0      up down
ae0.0     up down eth-switch

Log error messages:

dcd[2198]: DCD_INFO_MSG: MIXMODE : IFDSPF_AE_MIX_RATE is unset, IFDSPF_AE_10GE_MIX_MODE is unset for IFD ae0

dcd[2198]: DCD_INFO_MSG: MIXMODE : ifd(ae0), flags: is_valid 0, mix_rate_support 0 mix_configured 0

dcd[2198]: DCD_INFO_MSG: MIXMODE : IFDSPF_AE_MIX_RATE is unset, IFDSPF_AE_10GE_MIX_MODE is unset for IFD ae0

kernel: lag_ifdconfig_change: IFD ae0 ifd_specific_flag 0x0 changed to 0x0 by process dcd pid 2198

kernel: kernel did not add link ae4, link speeds differ (Child : 1000000000) and (Parent : 2) 

kernel: bundle ae4.0: link ge-0/0/4 not added due to speed mismatch

kernel: kernel did not add link ae4, link speeds differ (Child : 10000000000) and (Parent : 2)

kernel: bundle ae4.0: link xe-1/0/0 not added due to speed mismatch

The mixed rate of link speeds for the aggregated Ethernet bundle i.e. "link-speed mixed" knob is no longer supported on EX4600 for newer code releases 21.4R3-S7.

As per this document - link-speed (Aggregated Ethernet)
 

• mixed— (QFX5100, QFX100002, QFX100008, QFX100016 switches, MX Series, PTX Series routers) Enables bundling of different Ethernet rate links in the same Aggregated Ethernet interface.

• By default, the speed of the first aggregated interface that you add to the aggregated ethernet bundle determines the link speed of the aggregated ethernet interface bundle. If you specify the link speed, all the aggregated ethernet interfaces of the bundle operate at the specified speed.

• link-speed - On QFX5100 and EX4600 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.