Two physical controllers on the same network and same switch with a VIP configuration.

The VIP assigned to the irb interface takes on the mac address of the active controller device.

The problem is the ARP entry doesn't update and failover doesn't work.

When the controllers failover, the irb VIP in the router receives gratuitous-arps from the controller but the mac address of the VIP is not being updated with the MAC address of the active controller.

By default, updating the ARP cache on gratuitous ARP replies is disabled on the router or switch.

To enable updating of the ARP cache for gratuitous ARPs customer needs to use the knob “gratuitous-arp-reply” under the irb interface.

Reference

Configuring Gratuitous ARP

https://www.juniper.net/documentation/us/en/software/junos/multicast-l2/topics/task/interfaces-configuring-gratuitous-arp.html