Setup Guide

Polarization
Multipath routing is a method that is often used to address data forwarding issues during network failures so that the network trac
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 trac over multiple paths towards its destination. In a multi-tier network where
load balancing is performed at each tier, static hash algorithms polarize the trac where load balancing is ineective in the higher tiers. The
polarization eect is exaggerated if all the nodes in the network have to choose from the same set of ECMP paths. Trac polarization
results in packet reordering and route apping. The following gure explains the trac polarization eect. Router B performs the same hash
as router A and all the trac goes through the same path to router D, while no trac is redirected to router E. The following gure explains
the trac polarization eect:
Figure 39. Before Polarization
Eect
Router B performs the same hash as router A and all the trac goes through the same path to router D, while no trac is redirected to
router E.
Some of the anti-polarization techniques used generally to mitigate unequal trac distribution in LAG/ECMP as follows:
1 Conguring dierent hash-seed values at each node - Hash seed is the primary parameter in hash computations that determine
distribution of trac among the ECMP paths. The ECMP path can be congured dierent in each of the nodes “hash-algorithm
seed-value” would result in better trac distribution for a given ow, by reducing Polarization eect.
2 Conguring Ingress port as an additional load-balancing parameters [using “load-balance ingress-port enable”] would reduce the
polarization eect.
3 Conguring dierent load-balancing parameters at each tier. In Router A, the hash elds for load balancing could be source-ip, dest-ip,
vlan, protocol, L4-source-port and L4-dest-port, whereas on Router B, the hash elds use only source-ip, dest-ip, and protocol
4 Conguring dierent hash algorithms at dierent tiers. For example, Router A could use crc16 as the hash algorithm while router B
can use XOR16 as the hash algorithm.
Conguration and Benets
Equal Cost Multi-Path (ECMP)
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