Default PC/VPC Load-Balancing Algorythm

[julian.bendix]

Hey everybody,

I am trying to get a definitive answer on this question - which load-balancing algorythm is used by ACI Leaf Switches towards Endpoints when configuring a LACP Port-Channel.

You can only configure the Load-Balancing Method if you configure "Symmetric Hashing" ..
If Symmetric Hashing is NOT enabled (which is the default) - which mode is used?

I found that there is no such command like "show port-channel load-balancing", which we have on Nexus Switches.

I found this, which confused me a bit:
https://www.cisco.com/c/en/us/td/docs/switches/datacenter/aci/apic/sw/4-x/aci-fundamentals/Cisco-ACI-Fundamentals-42x/Cisco-ACI-Fundamentals-41X_chapter_0101.html?bookSearch=true
Here it says the defaults are not configurable (but they actually are once you enable Symmetric Hashing), and are like mentioned in the table.

But for IP Traffic towards end hosts, it mentions a "System generated random number" .. does this mean the TCP or UDP flow will NOT be forwarded on the exact same link, but load-balanced on all links? 

Sorry, I am quite confused, any input is much appreciated  

Thanks and best regards
Jules

[chandlerr]

Did you find anything?

[RedNectar]

Hi @julian.bendix & @chandlerr ,

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I am trying to get a definitive answer on this question - which load-balancing algorythm is used by ACI Leaf Switches towards Endpoints when configuring a LACP Port-Channel.

You can only configure the Load-Balancing Method if you configure "Symmetric Hashing" ..
If Symmetric Hashing is NOT enabled (which is the default) - which mode is used?

You can see that the default load balancing algorithm used by Cisco ACI when LACP is configured is in fact adaptive by issuing the following command from an APIC:

apic1# fabric <leaf_id> show port-channel internal info interface port-channel <port_channel_number> | grep hash
port-channel port hash-distribution : adaptive

To find out what adaptive really means, you'll have to rely on related documents rather than ACI specific documents.  Here's a couple of examples:

From: https://www.cisco.com/c/en/us/td/docs/ios/cether/configuration/guide/ce_lnkbndl.html

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The Load Distribution in an EtherChannel feature enhances the load distribution mechanism with the adaptive load distribution algorithm. This algorithm uses a port reassignment scheme that enhances EtherChannel availability by limiting the load distribution reassignment to the port that is added or deleted. The new load on existing bundled ports does not conflict with the load programmed on those ports when a port is added or deleted.

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From: a Nexus switch configuration guide

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Cisco NX-OS supports the adaptive and fixed hash distribution configuration for both global and port-channel
levels. This option minimizes traffic disruption by minimizing Result Bundle Hash (RBH) distribution changes
when members come up or go down so that flows that are mapped to unchange RBH values continue to flow
through the same links. The port-channel level configuration overrules the global configuration. The default
configuration is adaptive globally, and there is no configuration for each port channel, so there is no change
during an ISSU. No ports are flapped when the command is applied, and the configuration takes effect at the
next member link change event. Both modes work with RBH module or non-module schemes.

[Odetayo Adelabu]

My understanding:

Hashing will be done for each traffic (UDP&TCP) since ACI default hashing algorithm uses Src MAC/IP/L4port/Protocol to Dst MAC/IP/L4port/Protocol. The result of the hash for UDP traffic might result in the same portchannel member chosen as for TCP traffic or might not. However there is great chance that once these parameters (MAC/IP/L4port/Protocol) are changing for each  traffic flow, you will notice "load balancing". ACI Leaf will always SEND traffic flow with same Src MAC/IP/L4port/Protocol to same Dst MAC/IP/L4port/Protocol over the same Porchannel member.

https://www.cisco.com/c/en/us/td/docs/dcn/aci/apic/5x/layer-2-configuration/cisco-apic-layer-2-networking-configuration-guide-52x/access-interfaces-52x.html#:~:text=Port%20Channels-,PC/vPC%20Host%20Load%20Balancing%20Algorithms,-The%20following%20tab...