Which of the following statements about resilient ECMP load balancing is TRUE?

Resilient ECMP is an enhancement to standard Equal-Cost Multi-Path (ECMP) designed to minimize service disruption when a next-hop path fails or is restored. In traditional ECMP, a change in the number of available paths (e.g., from N to N-1) forces a recalculation of the hash algorithm for all flows, potentially re-routing traffic that was on stable paths. Resilient ECMP avoids this by using a more stable mapping mechanism. When a next-hop fails, only the traffic flows that were actively using that specific failed path are re-distributed among the remaining active next-hops. Flows on other, unaffected paths are not impacted, thus preserving their state and preventing unnecessary disruption.

A. This describes the hashing mechanism for standard, non-resilient ECMP. Resilient ECMP uses a more sophisticated method, often involving buckets, to avoid the complete flow re-shuffling caused by a simple modulo operation when the number of paths changes.
B. The goal described is to avoid hash polarization across multiple routers, which is a different network design consideration from resilience to a single next-hop failure on one router. Resilient ECMP's primary function is local failure recovery.
C. Using a different hash seed on downstream routers is a technique to prevent hash polarization, ensuring better traffic distribution across a network. This is not the defining characteristic or mechanism of resilient ECMP.

1. Nokia 7750 SR OS Services Architecture Guide, Release 21.10.R1, Section: Resilient ECMP: "When a next-hop is removed from the ECMP set, only the flows that are mapped to that next-hop are redistributed... Flows mapped to the other next-hops of the ECMP set are not modified." (Page 388)

2. Nokia 7750 SR OS Routing Protocols Guide, Release 21.10.R1, Section: ECMP Hashing: This section contrasts the behavior of the default hashing algorithm with resilient ECMP, explaining that the default algorithm re-hashes all flows upon a path change, while resilient ECMP provides stability for unaffected flows. (Page 124)