Etherchannel load balancing from 6500 to 3750, output drops

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Feb 22nd, 2010
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Having a hard time finding a definitive answer or method of testing.  At a remote site, I have dual fiber connections bundled in an etherchannel/port-channel between my C6506 (upstream) and my C3750 (downstream).  Lately I've noticed that one of the links heading towards the 3750 has been experiencing output drops.  The C6506 isn't doing any type of L3 routing, but the method of load-balancing is "src-dst-ip", whereas the 3750 is "src-mac".

At first I though the C6506 might not be properly load-balancing at all, and simply forcing the traffic over one of the two link, since the citeria is "ip" based.  That would explain the output drops.  But, no.  The counters were cleared 4 days ago and I see significant traffic over both links, so some type of balancing if happening.

My question is:  From a L2 perspective, what's the best way of isolation the cause of the output drops?

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Giuseppe Larosa Mon, 02/22/2010 - 23:48
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Hello Omegatek1,

whatever algorithm you use it is not per packet but flow based.

C6500 also on a L2 etherchannel uses srcIP exor destinationIP for IP flows this should not be a problem.

So if few very high traffic volume flows exist and are mapped to the same link the risk of saturation of that member link exists.

Hope to help



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