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Hash algorithm
The load-balance command discussed above selects the hash criteria applied to port channels.
If even distribution is not obtained with the load-balance command, the
hash-algorithm command can be
used to select the hash scheme for LAG, ECMP and NH-ECMP. The 12 bit Lag Hash can be rotated or
shifted till 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.
For the E-Series TeraScale and ExaScale, you can select one of 47 possible hash algorithms (16 on
EtherScale).
Figure 20-19 shows a sample configuration for the
hash-algorithm command.
Figure 20-19. Command example: hash-algorithm
On C-Series and S-Series, the hash-algorithm command is specific to ECMP groups and has different
defaults from the E-Series. The default ECMP hash configuration is
crc-lower. This takes the lower 32 bits
of the hash key to compute the egress port. Other options for ECMP hash-algorithms are:
• crc-upper — uses the upper 32 bits of the hash key to compute the egress port
Command Syntax Command Mode Purpose
hash-algorithm {algorithm-number} |
{ecmp {checksum|crc|xor}
[number]} lag
{checksum|crc|xor][number]}nh-ec
mp
{[checksum|crc|xor] [number]}}|
{linecard number ip-sa-mask value
ip-da-mask value}
CONFIGURATION Change the default (0) to another algorithm and apply
it to ECMP, LAG hashing, or a particular line card.
Note: To achieve the functionality of hash-align
on the ExaScale platform, do not use CRC as an
hash-algorithm method. For ExaScale systems,
set the default hash-algorithm method to ensure
CRC is not used for LAG. For example,
hash-algorithm ecmp xor lag checksum nh-ecmp
checksum
For details on the algorithm choices, see the command
details in the IP Routing chapter of the FTOS
Command Reference.
Note: E-Series systems require the lag-hash-align microcode be configured in the in the CAM profile.
E-Series TeraScale e
t
includes this microcode as an option with the Default cam profile. E-Series
ExaScale e
x
systems require that a CAM profile be created and specifically include lag-hash-align
microcode.
FTOS(conf)#FTOS(conf)#hash-algorithm ecmp xor 26 lag crc 26 nh-ecmp checksum 26
FTOS(conf)#