The MC-LAG peers use Inter-Chassis Control Protocol (ICCP) to exchange control information and coordinate with each other to ensure that data traffic is forwarded properly.

Configure Interchassis Control Protocol (ICCP)

Example: Configuring Multichassis Link Aggregation

https://www.juniper.net/documentation/en_US/junos/topics/reference/configuration-statement/traceoptions-edit-protocols-iccp.html

jlk@ex4600-1> show configuration protocols iccp | display set
set protocols iccp local-ip-addr 172.16.196.1
set protocols iccp peer 172.16.196.2 session-establishment-hold-time 50
set protocols iccp peer 172.16.196.2 backup-liveness-detection backup-peer-ip 172.30.92.201
set protocols iccp peer 172.16.196.2 liveness-detection minimum-interval 500
set protocols iccp peer 172.16.196.2 liveness-detection multiplier 2