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ECMP hash seed

  • 22 November 2018
  • 2 replies
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Hello everyone,

Can someone clarify for me how the "hash seed" affect the hasing calculation and how it is helps avoid hash polarization?

thanks 🙂
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Best answer by Eric Pulvino 27 November 2018, 05:16

The hash seed is an integer used in the ECMP calculation. In a large homogenous network where all leafs shared the same seed value it would theoretically be possible to overwhelm individual spines when servicing hosts with somewhat precisely crafted traffic sent over numerous high-bandwidth links.

The above scenario is very unlikely as switchd will randomly generate a unique seed value per node. These values do not typically need modification. In multiple years of consulting with customers I've yet to encounter a scenario where modification of the seed value makes sense.
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The hash seed is an integer used in the ECMP calculation. In a large homogenous network where all leafs shared the same seed value it would theoretically be possible to overwhelm individual spines when servicing hosts with somewhat precisely crafted traffic sent over numerous high-bandwidth links.

The above scenario is very unlikely as switchd will randomly generate a unique seed value per node. These values do not typically need modification. In multiple years of consulting with customers I've yet to encounter a scenario where modification of the seed value makes sense.
Hi Eric,

Thank you 🙂

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