Our Lab Setup on the MX480 has dual route Engines with both the production version (22.2R3-S2.8) RE1 & (23.4R2-S3.9) RE0 to efficiently test Upgrade & downgrade.

MX 480 -

Current Production version: 22.2R3-S2.8

Upgrade version: 23.4R2-S3.9

Some of the P2P Untagged interfaces do not see ARP entries on RE Switchover from RE0 to RE1 , An FPC Restart on 22.2R3-S2.8 or a switchover back to RE0 (23.4R2-S3.9) fixes it.

Juniper Networks does not support Graceful Routing Engine Switchover (GRES) or Nonstop Routing (NSR) during a Junos OS upgrade, or when the two Routing Engines (REs) are operating with different versions of the Junos software.

This restriction is due to the fact that only a single version of firmware can be loaded onto a line card, and it must correspond to the operating system version of the active (master) Routing Engine. If the Routing Engines are running different OS versions, a mastership switchover would require the installation of a different firmware version on the line cards. Consequently, the line cards must be reloaded, causing service disruption and preventing a seamless transition.