BFD sessions will not come up for LAG member interfaces.
root@> show bfd session detail
Detect Transmit
Address State Interface Time Interval Multiplier
206.253.84.32 Down et-0/0/0 0.000 1.000 3
Client LACPD, TX interval 0.300, RX interval 0.300
Local diagnostic None, remote diagnostic None
Remote state AdminDown, version 1
Replicated
Session type: Micro BFD
1 sessions, 1 clients
Cumulative transmit rate 1.0 pps, cumulative receive rate 0.0 pps
BFD over child links of AE or LAG bundle is not an officially supported feature on QFX5120 devices.https://apps.juniper.net/feature-explorer/parent-feature-info.html?pFKey=1161&pFName=Bidirectional+Forwarding+Detection+%28BFD%29
https://apps.juniper.net/feature-explorer/parent-feature-info.html?pFKey=1161&pFName=Bidirectional+Forwarding+Detection+%28BFD%29