Dell Configuration Guide for the S4048–ON System 9.14.2.5

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:

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 dynamic hash function selection across different nodes in a network on a macro flow basis, thus reducing unfair distribution of bandwidth between members and starvation.

Selection of Algorithms is available under flow-based-hashing enabling another level of randomness in hash selection. CLI to enable flow-based hashing is shown in the given example:

Dell_GW1(conf)#hash-algorithm ecmp flow-based-hashing ?
crc16                   CRC16_BISYNC - 16 bit CRC16-bisync polynomial(default)
crc16cc                 CRC16_CCITT - 16 bit CRC16 using CRC16-CCITT polynomial
crc32MSB                CRC32_UPPER - MSB 16 bits of computed CRC32
crc32LSB                CRC32_LOWER - LSB 16 bits of computed CRC32
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]

Example to view show hash-algorithm:

DellEMC(conf)#hash-algorithm ecmp flow-based-hashing crc16
DellEMC(conf)#end
DellEMC#show hash-algorithm

Hash-Algorithm

linecard 0 Port-Set 0
Seed 185270328
Hg-Seed 185282673
EcmpFlowBasedHashingAlgo- crc16
EcmpAlgo- crc32MSB       LagAlgo- crc32LSB  HgAlgo- crc16

Traffic flow after enabling flow-based hashing

When the flow-based hashing is enabled at all the nodes in the multi-tier network, traffic distribution is balanced at all tiers of the network nullifying the polarization effect. Traffic occurs by the randomness for the flow-based hashing algorithm across multiple nodes in a given network.