White Papers

ipv6-selection Set the IPV6 key fields to use in hash computation(default =
source-ipv6 dest-ipv6 vlan protocol L4-source-port L4-dest-port)
mac Set the mac key fields to use in hash computation(default =
source-mac dest-mac vlan ethertype)
tcp-udp Option to use TCP/UDP ports in packet for ECMP/LAG hashing
tunnel Set the tunnel key fields to use in hash computation(default
= 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.
DellEMC(conf)#hash-algorithm ecmp ?
crc16 CRC16_BISYNC - 16 bit CRC16-bisync polynomial
crc16cc CRC16_CCITT - 16 bit CRC16 using CRC16-CCITT polynomial
crc32LSB CRC32_LOWER - LSB 16 bits of computed CRC32
crc32MSB CRC32_UPPER - MSB 16 bits of computed CRC32(default)
crc-upper Use Upper 32 bits of key for hash computation
flow-based-hashing Enable flow based hashing
dest-ip Use Destination IP for ECMP hashing
lsb Always return the LSB of the key as the hash
xor1 CRC16_BISYNC_AND_XOR1 - Upper 8 bits of CRC16-BISYNC and
lower 8 bits of xor1
xor2 CRC16_BISYNC_AND_XOR2 - Upper 8 bits of CRC16-BISYNC and
lower 8 bits of xor2
xor4 CRC16_BISYNC_AND_XOR4 - Upper 8 bits of CRC16-BISYNC and
lower 8 bits of xor4
xor8 CRC16_BISYNC_AND_XOR8 - Upper 8 bits of CRC16-BISYNC and
lower 8 bits of xor8
xor16 CR16 - 16 bit XOR]
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:
Equal Cost Multi-Path (ECMP)
315