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Dell EMC Networking OS Configuration Guide for the S5048F–ON System 9.14.2.8

Changing the Hash Algorithm

The load-balance command selects the hash criteria applied to port channels.

If you do not obtain even distribution with the load-balance command, you can use the hash-algorithm command to select the hash scheme for LAG, ECMP and NH-ECMP. You can rotate or shift the 12–bit Lag Hash until the desired hash is achieved.

The nh-ecmp option allows you to change the hash value for recursive ECMP routes independently of non-recursive ECMP routes. This option provides for better traffic distribution over available equal cost links that involve a recursive next hop lookup.

To change to another algorithm, use the second command.

  • Change the default (0) to another algorithm and apply it to ECMP, LAG hashing, or a particular line card.
    CONFIGURATION mode
    hash-algorithm {algorithm-number | {ecmp {crc16 | crc16cc | crc32MSB | crc32LSB | crc-upper | dest-ip | lsb | xor1 | xor2 | xor4 | xor8 | xor16} [number] hg {crc16 | crc16cc | crc32MSB | crc32LSB | xor1 | xor2 | xor4 | xor8 | xor16} stack-unit stack-unit-number | port-set port-pipe | hg-seed seed-value stack-unit | lag {checksum | crc | xor} [number] nh-ecmp {checksum | crc | xor}[number] stack—unit number ip-sa-mask value ip-da-mask value | seed seed-value }

    For more information about algorithm choices, refer to the command details in the IP Routing chapter of the Dell EMC Networking OS Command Reference Guide.

  • Change the Hash algorithm seed value to get better hash value

    Hash seed is used to compute the hash value. By default hash seed is chassis MAC 32 bits. we can also change the hash seed by the following command.

    CONFIGURATION mode
    hash-algorithm seed {seed value}
  • Change to another algorithm.
    CONFIGURATION mode
    hash-algorithm [ecmp{crc16|crc16cc|crc32LSB|crc32MSB|crc-upper|dest-ip|lsb|xor1|xor2|xor4|xor8|xor16}]

Example of the hash-algorithm Command

DellEMC(conf)#hash-algorithm ecmp xor 26 lag crc 26 nh-ecmp checksum 26
DellEMC(conf)#
The hash-algorithm command is specific to ECMP group. The default ECMP hash configuration is crc-lower. This command takes the lower 32 bits of the hash key to compute the egress port. Other options for ECMP hash-algorithms are:
  • crc16 — uses 16 bit CRC16-bisync polynomial
  • crc16cc — uses 16 bit CRC16 using CRC16-CCITT polynomial
  • crc32LSB — uses LSB 16 bits of computed CRC32
  • crc32MSB — uses MSB 16 bits of computed CRC32(default)
  • crc-upper — uses the upper 32 bits of the hash key to compute the egress port.
  • dest-ip — uses destination IP address as part of the hash key.
  • lsb — uses the least significant bit of the hash key to compute the egress port.
  • xor1 — uses Upper 8 bits of CRC16-BISYNC and lower 8 bits of xor1
  • xor2 — Upper 8 bits of CRC16-BISYNC and lower 8 bits of xor2
  • xor4 —Upper 8 bits of CRC16-BISYNC and lower 8 bits of xor4
  • xor8 — Upper 8 bits of CRC16-BISYNC and lower 8 bits of xor8
  • xor16 — uses 16 bit XOR.

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