What is Equal‑Cost Multi‑Path routing (ECMP) primarily used for in networks?

• A. Equal-Cost Multi-Path routing uses multiple paths with the same cost.
• B. Dynamic routing that selects a single best path and ignores backups.
• C. Routing based on differences in path metrics to prefer the shortest path only.
• D. Single-path routing with a backup default route.

Answer: (A)

Equal-Cost Multi-Path routing is about using several forwarding options to the same destination when those options have the same routing metric. When multiple next-hops share an equal cost, traffic can be split across them, increasing total throughput and providing redundancy if a link or path fails. In practice, routers balance traffic using load-sharing methods (such as per-flow or per-packet hashing) while keeping packets from the same flow on the same path to avoid out-of-order delivery. This is why the concept centers on multiple equal-cost paths rather than a single best path or paths with different costs. It’s not about a single path with a backup or about prioritizing the shortest path via differing metrics; it’s about leveraging multiple equal options to move more data and tolerate failures.