Reference Guide

326 | Equal Cost Multi-Path
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With 8 or less ECMPs, the ordering is lexicographic and deterministic. With more than 8 ECMPs, ordering
is deterministic, but it is not in lexicographic order.
Configurable 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 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.
In Figure 17-1, Core Router 1 is an E-Series TeraScale and Core Router 2 is an E-Series ExaScale. They
have similar configurations and have routes for prefix P with two possible next-hops. When Deterministic
ECMP is enabled and the hash algorithm and seed are configured the same, each flow is consistently sent
to the same next hop even though they are routed through two different chassis.
Task Command Syntax Command Mode
Enable IPv4 Deterministic ECMP Next Hop. ip ecmp-deterministic CONFIGURATION
Enable IPv6 Deterministic ECMP Next Hop. ipv6 ecmp-deterministic CONFIGURATION
Note: Packet loss might occur when you enable ip/ipv6 ecmp-deterministic for the first-time only.
Note: While the seed is stored separately on each port-pipe, the same seed is used across all CAMs.
Note: You cannot separate LAG and ECMP, but you can use different algorithms across 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.
Note: If the hash algorithm configuration is removed. Hash seed will not go to original factory default
setting.
Task Command Syntax Command Mode
Specify the hash algorithm seed.
hash-algorithm seed value [linecard number] [port-set number]
Range: 0 - 4095
CONFIGURATION