On QFX5100, MC-LAG may fail to work after upgrading from Junos OS 14.1X53 to 17.1R1. When this issue happens, it is likely due to the reason that the "redundancy-group-id-list" option is not configured under the "protocols iccp peer" hierarchy.

After the upgrade, ICCP peers fail to be established with the status below:

{master:0}
root@LEAF1> show iccp
 
Redundancy Group Information for peer 192.168.100.1
  TCP Connection       : In progress   <<<<<<<<<<<<<<<
  Liveliness Detection : Down
Backup liveness peer status: Down
Client Application: lacpd
Client Application: l2ald_iccpd_client

On QFX5100 with Junos OS 14.1X53, redundancy-group-id-list is an optional and hidden command, whereas with Junos OS 17.1R1, it is a mandatory command which needs to be added properly.
 

If similar issue occurs after upgrading to Junos OS 17.1R1, try to add " redundancy-group-id-list " option on both mc-lag peers with the following commands.

Example:

{master:0}[edit]
user@LEAF0# set protocols iccp peer 192.168.100.2 redundancy-group-id-list 1

{master:0}[edit]
user@LEAF2# set protocols iccp peer 192.168.100.1 redundancy-group-id-list 1

Or before the upgrade, add the above commands proactively.

Note: On Junos OS 14.1X53, "redundancy-group-id-list" is a hidden command, so input the entire completely. You may see a warning like below after adding such command on Junos OS 14.1X53 which is expected and does not affect the functionality or upgrade.

root@LEAF2# show protocols iccp
local-ip-addr 192.168.100.1;
peer 192.168.100.2 {
    session-establishment-hold-time 100;
    ##
    ## Warning: value redundancy-group-id-list ignored: unsupported platform (qfx5100-48s-6q)
    ##
    redundancy-group-id-list 1;
    backup-liveness-detection {
        backup-peer-ip 172.27.113.75;
    }
    liveness-detection {
        minimum-interval 1000;
        multiplier 3;
    }
}

• redundancy-group-id-list was also a hidden open on the initial code for QFX10K, but a commit check did not allow the commit through.

• "redundancy-group-id-list" :  only supports one, i.e. redundancy-group-id-list [ x ]. Multiple redundancy ids are not supported, i.e., redundancy-group-id-list [ x  y]. There are commit checks in the newer Junos OS codes. However, in the older code, there is not a commit check.