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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
ECMP for flow-based affinity includes link bundle monitoring.
Enabling Deterministic ECMP Next Hop
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.
NOTE: Packet loss might occur when you enable ip/ipv6 ecmp-deterministic for the first-
time only.
• Enable IPv4 Deterministic ECMP Next Hop.
CONFIGURATION mode.
ip ecmp-deterministic
• Enable IPv6 Deterministic ECMP Next Hop.
CONFIGURATION mode.
ipv6 ecmp-deterministic
Configuring the Hash Algorithm Seed
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.
The system provides a command line interface (CLI)-based solution for modifying the hash seed to
ensure that on each configured system, the ECMP selection is same. When configured, the same seed is
set for ECMP, LAG, and NH, and is used for incoming traffic only.
Equal Cost Multi-Path (ECMP)
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