The customer's IP VRF prefixes are reachable via equal-cost multi-path (ECMP) core links. They have migrated their MPLS transport protocols from LDP / RSVP, configured with link protection,
to segment routing (L-ISIS) without link protection, but only with ECMP.

After the migration, they started to receive complaints of packet loss upon one or more of the ECMP member's interface flaps

 

Suggested to the customer to enable ecmp-fast-reroute which should enable fast reroute protection for ECMP links and minimize the packet loss during link failure.

ECMP-FRR is not enabled by default it's not enabled by default in the MX platform ( only enabled by default in PTX )

ECMP is a global configuration and will impact only the next-hop addresses learned after the ECMP configuration.
However, the existing next-hop addresses are not impacted.

for ECMP-FRR in the existing next-hop to take effect after the configuration is applied,

We need to perform either one of the below steps: 

1. Node reboot (recommended way) 
2. Restart line-cards
3. Routing process restart
4. Clear all ISIS/OSPF adjacencies/neighborships
5. Remove the ECMP from the node by manipulating the cost/metric so that all the routes prefer one exit interface