Port Channel Load Balancing Issue – Inconsistent Output Interface

[elahe]

Dear Community members,

I have a question on the following command on 9k Nexus switches but I could not find any documentation for it. Can anybody please explain me what it is and how it works or provide a reference for it please. Any guidance is appreciated.

port-channel load-balance internal ....

It seems that the default value of this parameter is dlb. But I could not find anything on this "internal" command and what it does.

In more details, I have a pair of switches configured as a single Port Channel using vPC. However, I am facing an issue where packets from the same flow (same source/destination IP and port) arriving at each switch are being forwarded through different output interfaces. For example:

• Switch 1 consistently forwards the traffic via Ethernet1

• Switch 2 consistently forwards the traffic via Ethernet3

This behavior is not desirable in my scenario. I need each flow, even if its packets arrive at different switches from outside of my network, to always exit through the same interface to ensure a consistent path within my network.

Since both switches use the same PO hashing algorithm (ip-l4port with the same rotate value), I suspect dynamic load balancing might be influencing this behavior. Could this be the root cause? Is there anyway I can ensure that each flow follows a single egress path?

Any guidance or best practices for handling this situation would be greatly appreciated.

Thank you in advance!

[marce1000]

 - Could you try command-completion-help as in : port-channel load-balance ?
                            Sometimes the options are then listed with a short  explanation,

 M.

[elahe]

Thank you but unfortunately the internal switch help dose not provide any information:

SW(config)# port-channel load-balance ?
dst Destination based parameters
internal Configure port-channel load balance internal commands
resilient Configure port-channel load balance resilient mode
src Source based parameters
src-dst Source-destination based parameters

SW(config)# port-channel load-balance internal ?
dynamic-pin Configure port-channel load balance internal commands
rtag7 Configure port-channel load balance internal commands

[marce1000]

 - Do you observe any traffic pattern differences between :
                             SW(config)# port-channel load-balance internal dynamic-pin
               or            SW(config)# port-channel load-balance internal rtag7

  M.

[Joseph W. Doherty]

Sorry, I'm unsure I fully understand.  Is the issue that once packets hit one of the vPC members, its egress traffic always uses the locally physical link, or are you saying if there's four links in the vPC, two per Nexus, egress doesn't use the same physical port on each Nexus?

[elahe]

Sorry if I couldn't explain well. The second one. Consider two nexus switches each with 5 egress interfaces for example.

One packet with same ip and port addresses (or even the result of sho vpc forwarding-path with same ip and port) is forwarded through different interface. For example sw1 forwards it through eth1 and sw2 through eth4.

All port channel parameters including the load balance algorithm, rotate value, etc are the same in both switches. 

[sdroy]

The `port-channel load-balance internal` command on Nexus 9k switches configures internal load balancing within port channels, and its default value dynamic load balancing might influence traffic distribution. In your scenario, where vPC-connected switches forward packets of the same flow via different interfaces, this could result from dynamic load balancing.

To ensure consistent egress paths, you can confirm that both switches use the same hashing algorithm and rotate value, consider switching to a static load balancing method, or enable features like symmetric or resilient hashing if supported. These steps can help maintain flow consistency within your network, avoiding the unpredictability observed in dynamic balancing setups.

[elahe]

Thanks a million. Can you please explain it more. What is internal load balancing? Is it flow control? How can I change it?

How can I use static load balancing?

Both switches are using same hash algorithm. I tried both ip and ip l4 port algorithms and none worked fine. The rotate value is the same and symmetric option is also enabled. But the problem exists.

Can you please guide me through resilient hashing? I used fixed hash distribution but it didn't solve the issue either. Is resilient hashing different from fixed hash distribution? How can it resolve the issue?

Thank you very much for your time and attention