Dell Force10 S4810P, Reference Manual

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Usage
Information 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 means that for a given flow, even though the
prefixes are sorted, two unrelated chassis will select different hops.
FTOS provides a 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.
NOTE: While the seed is stored separately on each port-pipe, the same seed is used
across all CAMs.
You cannot separate LAG and ECMP but you can use different algorithms across the
chassis with the same seed. If LAG member ports span multiple port-pipes and line cards,
set the seed to the same value on each port-pipe to achieve deterministic behavior.
If the hash algorithm configuration is removed, the hash seed does not go to the original
factory default setting.
ip ecmp-deterministic
Deterministic ECMP Next Hop arranges all ECMPs in order before writing them into the CAM.
E-Series
Syntax ip ecmp-deterministic
Defaults Disabled
Command Modes CONFIGURATION
Command History
Version 8.3.1.0 Introduced on the E-Series.
Usage
Information
Deterministic ECMP Next Hop arranges all ECMPs in order before writing them into the CAM.
For example, suppose the RTM learns eight ECMPs in the order that the protocols and
interfaces came up. In this case, the 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.
After enabling IPv6 Deterministic ECMP, traffic loss occurs for a few milliseconds while FTOS
sorts the CAM entries.
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