Administrator Guide
Equal Cost Multi-Path (ECMP)
Equal cost multi-path (ECMP) supports multiple paths in next-hop packet forwarding to a destination device.
ECMP for Flow-Based Anity
ECMP for ow-based anity includes link bundle monitoring.
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 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.
The system provides a command line interface (CLI)-based solution for modifying the hash seed to ensure that on each congured system,
the ECMP selection is same. When congured, the same seed is set for ECMP, LAG, and NH, and is used for incoming trac 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 dierent algorithms across the 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 you remove the hash algorithm conguration, the hash seed does not return to the original factory default
setting.
To
congure the hash algorithm seed, use the following command.
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