Dell Force10 E300, Configuration Manual, Page: 332 / 1262

332 | Equal Cost Multi-Path
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For information on the load-balancing criteria used by the hash algorithm to distribute traffic among
ECMP paths and LAG members on an E-Series system, see
E-Series load-balancing on page 436 .
Deterministic ECMP Next Hop
Deterministic ECMP Next Hop arranges all ECMPs in order before writing them into the CAM. For
example, suppose the RTM learns 8 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 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.
FTOS Behavior: In FTOS versions prior to 8.2.1.2, the ExaScale default hash-algorithm is 0.
Beginning with version 8.2.1.2, the default hash-algorithm is 24.
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.