Three port etherchannel issue

Unanswered Question
Sep 17th, 2009

Hi,

I am using 3 physical ports in each core 6509 E switch for the etherchanel. Always first port of the 3 ports shows high utilization.What cause this problem?

swami

I have this problem too.
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glen.grant Thu, 09/17/2009 - 03:46

You probably have a lot of traffic going to one particular address or network so as the other poster said try altering the load balancing scheme . A given conversation say to a given ip address will always flow down "one port in the channel" due to the hashing algorithims the switch uses to balance the traffic , try one of the others.

Joseph W. Doherty Thu, 09/17/2009 - 04:54

To amplify what the other posters have already described, regardless of hash being used, single flows only use one port and if attributes of flow are always the same, they will use the same port (assuming number of ports in channel doesn't change).

This means, if you have some very heavy backup application or replication application between a pair of hosts, it will use just one port and can skew your load stats.

Selection of hasing algorithm can very much impact load distribution for many different flows, depending on flow attributes. You must be very careful if using an algorithm that depends on only one source or destination attribute, and for these, you might need to define different hash algorithms per flow's direction. (Hash algorithms that use both souce and destination attributes can generally be used for both directions.)

One last minor point, hashing works out binary values which means even "ideal" hashing across multiple ports only works best for a binary multiple number of ports. I.e. 3 ports wouldn't balance out as well as 2 or 4.

PS:

BTW, assuming you can perform routing between L3 switches, you might consider CEF which, I believe, will round-robin flows per path. (This for the non-packet-by-packet. Packet-by-packet CEF might provide the best load balancing but makes for other issues; i.e. normally best to avoid.)

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