Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.14.1.0 - Page 324

ipv6-over-ipv4 Payload header ipv4-over-gre-ipv4 Payload header ipv6-over-gre-ipv4 Payload header ipv4-over-gre-ipv6 Payload header ipv6-over-gre-ipv6 Payload header mac-in-mac header based hashing is disabled TcpUdp Load Balancing Enabled DellEMC(conf)# • 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. DellEMC(conf)#load-balance ? flexhash Enable flexhash based on IP Protocol ingress-port Option to Source Port Id for ECMP/LAG hashing ip-selection Set the IPV4 key fields to use in hash computation(default = source- ip dest-ip vlan protocol L4-source-port L4-dest-port) 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: 324 Equal Cost Multi-Path (ECMP)