For EX4300 switch, Applied Junos upgrade from 20.4R3-S3.4 to 21.4R3-S5.4 post upgrade and the interfaces are not coming up and the state remains in down / down.

When an upgrade is completed successfully and the ports are not coming UP it may be an indication of a commit failed.

When the upgrade is completed, the system runs a commit and if there is any conflict, the configuration is not committed causing the interfaces to stay down.

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NOTE: The possibility also exists that the configuration may pass and get commited, however the error exists but won't show up until you may perform any change and then try to commit the configuration. If your commit is passing normally, try to make a minimum change in the configuration, like deleting something, then try a commit check and you will see the error.

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The following error could be observed:

master:0}[edit]

user@switch# commit check

error: Address cannot be configured on me0 and vme at the same time

error: configuration check-out failed

The previous output reports a conflict with me0 and vme.

When checking the configuration, found that both interfaces are configured. 

This was allowed on 21.4R3-S3, but is not allowed any more on 21.4R3-S5.

Check the configuration for the following statement:

{master:0}[edit]

user@switch# show | display set | match me0

set interfaces me0 unit 0 family inet address <IP address>

{master:0}[edit]

user@switch# show | display set | match vme

set interfaces vme unit 0 family inet address <IP address>

On Juniper EX4300 switches, me0 is the physical out-of-band management Ethernet interface (typically an RJ-45 port), while vme.0 (or vme) is the virtual management Ethernet interface used for consistency, especially in Virtual Chassis setups where it maps to the me0 port.

Key Differences:

• me0: Dedicated physical port for out-of-band management on standalone switches. It's directly accessible and used for connecting to management networks. Factory-default configurations often disable IGMP on me0.0.
• vme.0: Virtual interface that provides a consistent management access point. In a Virtual Chassis (VC), vme is accessed via the me0 port across members. It can be configured on standalone EX4300 switches for uniformity and is supported in non-default VRF instances for Virtual Chassis management.

Therefore there is no need to keep both at the same time configured, that is the commit error observed.

Removing the configuration of the one not needed at the moment, that is causing the conflict, will solve the commit issue, and after commit the ports will come UP.

Please have a look at this KB: https://supportportal.juniper.net/s/article/MGE-ports-down-after-reboot