Service Manual
14
Equal Cost Multi-Path (ECMP)
This chapter describes conguring ECMP.
ECMP for Flow-Based Anity
Flow-based anity includes the following:
• Link Bundle Monitoring
Conguring the Hash Algorithm
TeraScale has one algorithm that is used for link aggregation groups (LAGs), ECMP, and NH-ECMP, and ExaScale can use three
dierent 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 sorts them so that the ECMPs are always arranged. This implementation ensures that
every chassis having the same prexes 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 rst-time only.
• Enable IPv4 Deterministic ECMP next hop.
CONFIGURATION mode.
ip ecmp-deterministic
• Enable IPv6 Deterministic ECMP next hop.
CONFIGURATION mode.
ipv6 ecmp-deterministic
Conguring 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 dierent hash result for every chassis.
This behavior means that for a given ow, even though the prexes are sorted, two unrelated chassis can select dierent hops.
256
Equal Cost Multi-Path (ECMP)