398 | Equal Cost Multi-Path (ECMP)w w w . d e l l . c o m | s u p p o r t . d e l l . c o mDeterministic ECMP Next HopDeterministic ECMP Next Hop arranges all ECMPs in order before writing them into the CAM. Forexample, suppose the RTM learns 8 ECMPs in the order that the protocols and interfaces came up. In thiscase, the FIB and CAM sort them so that the ECMPs are always arranged.This implementation ensures thatevery 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, orderingis deterministic, but it is not in lexicographic order.Configurable Hash Algorithm SeedDeterministic 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 resultfor every chassis. This means that for a given flow, even though the prefixes are sorted, two unrelatedchassis will select different hops.FTOS provides a CLI-based solution for modifying the hash seed to ensure that on each configuredsystem, the ECMP selection is same. When configured, the same seed is set for ECMP, LAG, and NH, andis used for incoming traffic only.FTOS Behavior: In FTOS versions prior to 8.2.1.2, the ExaScale default hash-algorithm is 0. Beginning withversion 8.2.1.2, the default hash-algorithm is 24.Task Command Syntax Command ModeEnable IPv4 Deterministic ECMP NextHop.ip ecmp-deterministic CONFIGURATIONEnable IPv6 Deterministic ECMP NextHop.ipv6 ecmp-deterministic CONFIGURATIONNote: 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 thesame seed. If LAG member ports span multiple port-pipes and line cards, set the seed to the same valueon each port-pipe to achieve deterministic behavior.Note: If the hash algorithm configuration is removed. Hash seed will not go to original factory defaultsetting.Task Command Syntax Command ModeSpecify the hash algorithm seed. hash-algorithm seed value [linecard number][port-set number]Range: 0 to 4095.CONFIGURATION
