Etherchannel - issue

[aguante_boca]

Dear Fellows,

I have been experiencing some issue with a portchannel interface.

I have two cisco switches 3560 and I have a portchannel between them ( It is composed by two gigabitethernet interfaces generating a portchannel of 2 Gigas ).

Unfortunately, I am verifying that one interface traffics about 893MB but the other interface only traffics about 100MB.

Is there any cause why is this happening?

I thought that maybe both interfaces would traffic about 450MB.

Is there some wrong configuration with the load-balance command?. At this moment there is no load-balance, It is only configured the portchannel with the default configuration.

Thanks in advanced!

[Giuseppe Larosa · Accepted solution]

Hello Onno,

load balancing is flow based in etherchannel so what you see can happen without errors in configuration.

if there are few very high traffic volume flows  ( based on IP source address and IP destination address) and the hashing function maps them to the same member link, one link will be much more used then the other member link.

Hope to help

Giuseppe

[aguante_boca]

Thank Giuseppe!

However, I would like to know if there is any chance to balance both links correctly. (450MB more o less on both gigabitethernet interfaces).

Thanks in advanced!

[Edwin Summers · Accepted solution]

No.  As Giuseppe noted, you're limited by the hashing function.  Depending on the device's capabilities you can configure hashing to best relate to your particular flows, but you won't be able to perfectly balance the link.  I feel that Cisco Etherchannel "load balancing" should be better termed "load sharing" as it's not really "balancing" traffic.

[aguante_boca]

So, as I understand. If I continue aggregating more interfaces, only one interface is going to flow data correctly. Is that correct?.

The other ones are going to traffic only a 10%?

[Joseph W. Doherty]

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Posting

Not exactly.

A single traffic flow will only use one link.  Multiple flows are "hashed" across all the links.  As the hashing is static, it's possible flows will be directed to the same link even when other links are unused.  Additionally, Etherchannel doesn't account for how busy any link is, so a couple of high volume flows might saturate some link or links, while other links only have low volume flows and available capacity.

As mentioned by the other posters, some Cisco devices support multiple choices for the hash; sometimes changing hash being used can provide better load distribution (this always requires multiple flows to get this advantage).