Equal Cost Multi-Path (Ecmp); Ecmp For Flow-Based Affinity; Configuring The Hash Algorithm; Enabling Deterministic Ecmp Next Hop - Dell Force10 Z9000 Configuration Manual

Equal Cost Multi-Path (ECMP)

Equal cost multi-path (ECMP) is supported on Dell Networking OS.

ECMP for Flow-Based Affinity

Flow-based affinity includes the following:
• Link Bundle Monitoring

Configuring the Hash Algorithm

TeraScale has one algorithm that is used for link aggregation groups (LAGs), ECMP, and NH-ECMP, and ExaScale can use three
different algorithms for each of these features.
To adjust the ExaScale behavior to match TeraScale, use the following command.
• Change the ExaScale hash-algorithm for LAG, ECMP, and NH-ECMP to match TeraScale.
CONFIGURATION mode.
hash-algorithm ecmp checksum 0 lag checksum 0 nh-ecmp checksum 0
Dell Networking OS Behavior: In the Dell Networking OS versions prior to 8.2.1.2, the ExaScale default hash-algorithm is 0.
Beginning with Dell Networking OS version 8.2.1.2, the default hash-algorithm is 24.

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.
Equal Cost Multi-Path (ECMP)
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