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Nexus port channel load balance


I just want to clarify one setting for the port channel load balance on Nexus 6k switch. If I use the load balance option source-dest-ip-only, will following four converstions be load balanced? -> -> -> ->

Thanks. Leo


Re: Nexus port channel load balance

Hi Leo,

I think there may be typo in your question as I only see three conversations and not four. That aside I've not seen the Nexus port-channel load balancing sufficiently well documented to be able to give you the exact answer.

In their configuration guides Cisco only include the following statement:

Cisco NX-OS load balances traffic across all operational interfaces in a port channel by reducing part of the binary pattern formed from the addresses in the frame to a numerical value that selects one of the links in the channel.

There is other documentation that states the load balancing algorithm uses a CRC-8 based polynomial, but as we don't know exactly which parts of the frame are used in the calculation, I don't see it's possible to calculate the answer and so derive the link that will be used for a given conversation.

While I've not seen full documentation regarding the science used in the calculation, what Cisco have done is provide a command on the switch CLI that will allow you to determine which link of a port-channel will be used.

If you run the command show port-channel load-balance forwarding-path interface port-channel vlan src-ip dst-ip then one of the parts of the output is the member link of the port-channel that will be used for that flow.

You can find full details of the options for the show port-channel load-balance command in the command reference.

One other point to remember is that the load balancing across a port-channel is unidirectional, and the hashing might be completely different for the return flow of a conversation. For example it is entirely possible that traffic from A to B could use one link of a port-channel, while the return traffic from B to A for the same conversation could use a different link.

In general I would use the source-dest-port option for load balancing on the Nexus switches as this will obviously include the Layer-4 port numbers in the calculation, and so give you a better distribution of flows across all member links.



Re: Nexus port channel load balance

Hi Steve,

Thanks for youre reply and advices

Cheers, Leo

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