Equal Cost Multi-Path (Ecmp); Ecmp For Flow-Based Affinity; Enabling Deterministic Ecmp Next Hop; Configuring The Hash Algorithm Seed - Dell C9000 Series Networking Configuration Manual

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Equal Cost Multi-Path (ECMP)

Equal cost multi-path (ECMP) supports multiple paths in next-hop packet forwarding to a destination device.

ECMP for flow-based affinity includes link bundle monitoring.

Deterministic ECMP next hop arranges all ECMPs in order before writing them into the content addressable

memory (CAM).

For example, suppose the RTM learns eight ECMPs in the order that the protocols and interfaces came up. In

this case, the forwarding information base (FIB) and CAM sort them so that the ECMPs are always arranged.

This implementation ensures that every chassis having the same prefixes orders the ECMPs the same.

With eight or less ECMPs, the ordering is lexicographic and deterministic. With more than eight ECMPs,

ordering is deterministic, but it is not in lexicographic order.

To enable deterministic ECMP next hop, use the appropriate command.

Packet loss might occur when you enable ip/ipv6 ecmp-deterministic for the first-time

Enable IPv4 Deterministic ECMP Next Hop.

CONFIGURATION mode.

ip ecmp-deterministic

Enable IPv6 Deterministic ECMP Next Hop.

CONFIGURATION mode.

ipv6 ecmp-deterministic

Deterministic ECMP sorts ECMPs in order even though RTM provides them in a random order. However, the

hash algorithm uses as a seed the lower 12 bits of the chassis MAC, which yields a different hash result for

every chassis.

This behavior means that for a given flow, even though the prefixes are sorted, two unrelated chassis can

select different hops.

Equal Cost Multi-Path (ECMP)