Service Manual

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
4. Configuring different hash algorithms at different tiers. For example, Router A could use crc16 as the
hash algorithm while router B can use XOR16 as the hash algorithm.
Configuration and Benefits
The preceding anti-polarization techniques require some coordinated configuration of network nodes to
solve the problem and these techniques are not scalable when the number of tiers in the network is high.
Flow based hashing specifically addresses this using Macro flow-based Hash function. It facilitates a
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Equal Cost Multi-Path (ECMP)