Port Channel Symmetric Hashing Configuration on Nexus switches

Port Channel Symmetric Hashing on Nexus switches

To be able to effectively monitor traffic on a port channel, it is essential that each interface connected to a port channel receives both forward and reverse traffic flows.

Normally, there is no guarantee that the forward and reverse traffic flows will use the same physical interface. However, when you enable symmetric hashing on the port channel, bidirectional traffic is forced to use the same physical interface and each physical interface in the port channel is effectively mapped to a set of flows.

When symmetric hashing is enabled, the parameters used for hashing, such as the source and destination IP address, are normalized before they are entered into the hashing algorithm. This process ensures that when the parameters are reversed (the source on the forward traffic becomes the destination on the reverse traffic), the hash output is the same. Therefore, the same interface is chosen.

How to enable Symmetric Hashing

The optional symmetric keyword is used to enable or disable symmetric hashing. Symmetric hashing forces bi-directional traffic to use the same physical interface.

switch(config)# port-channel load-balance ethernet {[src-dst ip | src-dst ip-l4port] symmetric | crc-poly}

On Nexus 9K, NXOS 10.3, Only the following load-balancing algorithms support symmetric hashing:

  • src-dst ip
  • src-dst ip-l4port


On Nexus 7K, the following load-balancing algorithms support symmetric hashing:

  • src ip
  • dst ip rotate
  • dst ip
  • src ip rotate
  • src-dst ip
  • src ip-l4port
  • dst ip-l4port rotate
  • dst ip-l4port
  • src ip-l4port rotate
  • src-dst ip-l4port-vlan
  • dst ip-vlan
  • src ip-vlan rotate
  • src-dst ip-vlan
  • src l4port
  • dst l4port rotate
  • dst l4port
  • src l4port rotate
  • src-dst l4port
  • src mac
  • dst mac rotate
  • dst mac
  • src mac rotate
  • src-dst mac


This example shows how to configure symmetric hashing for port channels:

switch# configure terminal
switch (config)# port-channel load-balance ethernet source-dest-ip-only symmetric


References:

Cisco.com

For Nexus 7k

Bilel

Bilel

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