Preempt mode

When preempt mode is enabled (the default setting), a higher priority backup router can preempt a lower priority primary router. This can happen if the primary router fails, the backup router becomes the primary router, and the failed primary router restarts. Since the restarted router has a higher priority, if preempt mode is enabled, the restarted router replaces the current primary router becoming the new primary router. If preempt mode is disabled, a restarted router that has a higher priority would not take over as the primary router.

Based on RFC 3768 Section 5.3.4, "The priority value for the VRRP router that owns the IP address(es) associated with the virtual router MUST be 255 (decimal). VRRP routers backing up a virtual router MUST use priority values between 1-254 (decimal)." Therefore, in cases where preempt mode is disabled, but the priority is set to 255, the restarted unit will take over as the primary router.

To configure preempt mode in IPv4 VRRP:

config system interface
    edit <name>
        config vrrp
            edit <vrid>
                set preempt {enable | disable}
            next
        end
    next
end

To configure preempt mode in IPv6 VRRP:

config system interface
    edit <name>
        config ipv6
            config vrrp6
                edit <vrid>
                    set preempt {enable | disable}
                next
            end
        end
    next
end