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Resilient hashing on ECMP

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  • 1.  Resilient hashing on ECMP

    Posted 12-01-2021 05:43
    Hi, I have the need to implement resilient hashing on QFX5200 platform, "set forwarding-options enhanced-hash-key ecmp-resilient-hash", ECMP next-hops are stateful SRXes, so if one ECMP next-hop fails and new next-hop is added, I want sessions currently being routed to other SRXes are not impacted, according to the documentation, the above command seems to be able to achieve my goal.

    However I have one doubt about how this resilient-hash works, say  there are existing sessions being forwarded to ECMP next-hop A, B, C based on whatever hashing algorithm, now a new ECMP next-hop D is added to the group, obviously QFX will need to load balance traffic to D, but how does QFX  decide existing flows should ever be routed to D?  QFX5200 is not a stateful devices, I suppose it won't be able to keep any session info in its PFE, it won't know a session is terminated or not, right?

    Appreciate your response.