Dell S4048T Configuration Manual page 374

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:
Figure 40. Before 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.
Some of the anti-polarization techniques used generally to mitigate unequal traffic distribution in LAG/ECMP
as follows:
1 Configuring different hash-seed values at each node - Hash seed is the primary parameter in hash
computations that determine distribution of traffic among the ECMP paths. The ECMP path can be
configured different in each of the nodes "hash-algorithm seed-value" would result in better traffic
distribution for a given flow, by reducing Polarization effect.
2 Configuring Ingress port as an additional load-balancing parameters [using "load-balance ingress-port
enable"] would reduce the polarization effect.
3 Configuring different load-balancing parameters at each tier. In Router A, the hash fields for load
balancing could be source-ip, dest-ip, vlan, protocol, L4-source-port and L4-dest-port, whereas on
Router B, the hash fields use only source-ip, dest-ip, and protocol
Equal Cost Multi-Path (ECMP) 374