15Equal Cost Multi-Path (ECMP)This chapter describes configuring ECMP. This chapter describes configuring ECMP.ECMP for Flow-Based AffinityECMP for flow-based affinity includes link bundle monitoring.Configuring the Hash AlgorithmTeraScale has one algorithm that is used for link aggregation groups (LAGs), ECMP, and NH-ECMP, and ExaScale can use threedifferent 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 0Dell 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 HopDeterministic 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, theforwarding information base (FIB) and CAM sorts them so that the ECMPs are always arranged. This implementation ensures thatevery 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, butit 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-deterministicConfiguring the Hash Algorithm SeedDeterministic ECMP sorts ECMPs in order even though RTM provides them in a random order. However, the hash algorithm uses asa 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) 301
