This article helps to resolve the issue where the MC-AE interfaces are seen down. 

MC-AE interface is down in one of the peers, in some cases the issue is seen after the upgrade. 

user@switch> show interfaces terse | match ae1

et-0/0/48.0             up     up   aenet    --> ae1.0

ae1                     up     down

ae1.0                   up     down eth-switch

On the both the peers, the status-control was configured as active

Peer1:

#set interfaces ae1 aggregated-ether-options mc-ae status-control active

Peer2:

#set interfaces ae1 aggregated-ether-options mc-ae status-control active

After changing the status-control to standby on one of the peer the issue resolved.

Peer1:

#set interfaces ae1 aggregated-ether-options mc-ae status-control active

Peer2:

#set interfaces ae1 aggregated-ether-options mc-ae status-control standby

user@switch> show interfaces terse | match ae1

et-0/0/48.0             up     up   aenet    --> ae1.0

ae1                     up     up

ae1.0                   up     up eth-switch

2024-11-14 : Article Created

2024-11-25 : Article Validated

https://www.juniper.net/documentation/us/en/software/junos/mc-lag/topics/topic-map/examples-mc-lag.html