lo0.x interface is required under the routing instance when InLine BFD sessions are configured under the routing-instance  

InLine BFD sessions will be flapping continuously when lo0.x interface is not configured under routing-instance, this is also mandatory for PTX EVO. This is expected behavior for PTX family as of now because if lo0 interface is not configured the hostbound ddos filter is not attached to lo0 in forwarding path, because of this packets are not classified and steering does not happen.

InLine BFD sessions are the one's whose interval is 100 msec or lower.

Inline BFD sessions will be flapping continuously.

Please add the lo0.x with the family config (IP address is not required) and assign the lo0.x address to the routing-instance or global RI.

(If BFD is enabled for both IPV4 and IPV6, please configure both the families under lo0.x, if BFD is enabled for one of the family, corresponding family can be configured for lo0.x)

set interfaces lo0 unit 100 family inet

set interfaces lo0 unit 100 family inet6

set routing-instances TEST interface lo0.100

For PTX EVO:

set interfaces lo0 unit 0 family inet

set interfaces lo0 unit 0 family inet6