This article describes behaviour of OSPF with ECMP path available for prefixes.

• Four OSPF neighbors were configured between MX10004 and Arista switches connected via an L2 tunnel. 
• ECMP was configured on the MX10004, resulting in four available paths for prefixes.
• When one link between the MX10004 and an Arista switch went down, one OSPF neighbor became unreachable, leaving three available paths.
• Despite the lack of BFD configuration and an OSPF dead timer of 8 seconds, failover to other paths occurred within approximately 80 milliseconds.

Observation:

• ECMP employs a protective mechanism that pre-installs all paths on the forwarding table at the PFE level.
• The system continuously monitors the reachability of the next-hop and switches to an available path if needed, without involving the control plane.

ECMP Behavior:

• Equal-cost paths are identified based on routing metrics and hash algorithms.
• When an interface goes down, the event is prioritized and triggers reprogramming.
• Unilist notifies the system of the member down event and directly updates the forwarding table using cached data, avoiding a full member walk and rebuild.

Conclusion:

• The observed 80ms convergence time is attributed to the presence of other ECMP paths for the same destination.