to echo Krasi's comments, VRRP configuration is needed between the two MC-LAG peers for proper ARP synchronization. I
we ran into this same issue when doing MC-LAG with a router, and also with an L2-only MC-LAG pair that only needed an IP address for Management -- we even had to run VRRP there between them so proper ARP sync of the default gateway would work correctly -- kind of silly, but there you have it.
While Juniper's configuration requirements for MC-LAG seem overly complicated to me, it has been pretty stable for us on both EX9200 and QFX5100 implementations in both L2-only and L2/L3 gateway scenarios.
Thanks,
Will
