Flow-Based Hashing For Ecmp - Dell S4048–ON Configuration Manual

• Packet Header parameters for the first portion of the RTAG7 hash can be controlled. By default, all the listed parameters from
the Packet header are considered for hash computation. Few parameters [on demand] can be removed using the given CLIs.
Dell(conf)#load-balance ?
flexhash
ingress-port
ip-selection
source-ip dest-ip vlan protocol L4-source-port L4-dest-port)
ipv6-selection
source-ipv6 dest-ipv6 vlan protocol L4-source-port L4-dest-port)
mac
source-mac dest-mac vlan ethertype)
tcp-udp
tunnel
Hash-computation based on Inner Header)]
• The second portion comes from static physical configuration such as ingress and egress port numbers.
• RTAG7 hashing also provides options to select between multiple hash algorithms that would result in balanced traffic distribution
for various traffic patterns.
Dell(conf)#hash-algorithm ecmp ?
crc16
crc16cc
crc32LSB
crc32MSB
crc-upper
flow-based-hashing
dest-ip
lsb
xor1
bits of xor1
xor2
bits of xor2
xor4
bits of xor4
xor8
bits of xor8
xor16

Flow-based Hashing for ECMP

Flow-based hashing is one of RTAG7 hashing techniques to cater to ECMP routing in multi-tier networks. It addresses traffic
polarization issues by ensuring proper flow distribution between ECMP members in the higher layers of a multi-tier network. It
facilitates a dynamic hash function selection across different nodes in the network on a macro flow basis, by reducing route
starvation and the unfair distribution of bandwidth between members.
Polarization
Multipath routing is a method that is often used to address data forwarding issues during network failures so that the network traffic
reaches its desired destination. Multipath routing in IP networks is typically implemented using Equal-Cost Multipath (ECMP)
routing, which employs load balancing algorithms to distribute the traffic over multiple paths towards its destination. In a multi-tier
network where load balancing is performed at each tier, static hash algorithms polarize the traffic where load balancing is ineffective
in the higher tiers. The polarization effect is exaggerated if all the nodes in the network have to choose from the same set of ECMP
paths. Traffic polarization results in packet reordering and route flapping. The following figure explains the traffic polarization effect.
Router B performs the same hash as router A and all the traffic goes through the same path to router D, while no traffic is redirected
to router E. The following figure explains the traffic polarization effect:
Enable flexhash based on IP Protocol
Option to Source Port Id for ECMP/LAG hashing
Set the IPV4 key fields to use in hash computation(default =
Set the IPV6 key fields to use in hash computation(default =
Set the mac key fields to use in hash computation(default =
Option to use TCP/UDP ports in packet for ECMP/LAG hashing
Set the tunnel key fields to use in hash computation(default =
CRC16_BISYNC - 16 bit CRC16-bisync polynomial
CRC16_CCITT - 16 bit CRC16 using CRC16-CCITT polynomial
CRC32_LOWER - LSB 16 bits of computed CRC32
CRC32_UPPER - MSB 16 bits of computed CRC32(default)
Use Upper 32 bits of key for hash computation
Enable flow based hashing
Use Destination IP for ECMP hashing
Always return the LSB of the key as the hash
CRC16_BISYNC_AND_XOR1 - Upper 8 bits of CRC16-BISYNC and lower 8
CRC16_BISYNC_AND_XOR2 - Upper 8 bits of CRC16-BISYNC and lower 8
CRC16_BISYNC_AND_XOR4 - Upper 8 bits of CRC16-BISYNC and lower 8
CRC16_BISYNC_AND_XOR8 - Upper 8 bits of CRC16-BISYNC and lower 8
CR16 - 16 bit XOR]
Equal Cost Multi-Path (ECMP)
305