Junos OS Release Notes for MX Series
These release notes accompany Junos OS Release 21.4R1 for the MX Series. They describe new and changed features, limitations, and known and resolved problems in the hardware and software.
You can also find these release notes on the Juniper Networks Junos OS Documentation webpage, located at https://www.juniper.net/documentation/product/en_US/junos-os.