I have made two runs up this hill:
Our company has 6513 at core and 3750 stacks in closets and server farms. We had a single fiber run to each stack from the 6513. I wanted to run additional fiber to each stack and port-channel them.
It worked well for the 6513 to the wiring closets on floors. But the same set up didn't work so well for the stacks that connect the server farms.
The first attempt we began to see duplicate IP errors all over the place. I studied all the information I could get my hands on and concluded that one of the stacks had to have had some kind of loop condition. I searcched and never found anything but then too, I don't control what all the server admins plug in. At one point I found a hub on one switch with cables going all over the place. I pulled it out of the mix right away.
The next chance I got to try etherchannel, I hooked that switch up first. After several hours of all 4 links up and in the channel group there were no errors. I added one more stack. Still no errors.
But then the next thing I started to get calls about slow servers and they were on the two stacks that I had set up port channeling for. Not all servers on those farms had issues, but the ones that did have slowness issues were on the switches where I'd turned on either channel. BUT no errors still!
I had to turn it back of so that people could function but I'm at a loss to explain. In fact while I was undoing the port channel I learned the admin folks were rebooting their servers and the problem went away. But I don't know for sure if it went away 'cause they rebooted or 'cause I shut off port channel.
I do know the servers in question use MSload-balancing. Is there anything about the way MS load balances that might make port channeling adversely impact performance?
I just can't explain this. If I am getting no errors on the links or in the server logs, I'm pretty sure I don't have a loop. Wonder where else to look? I would love to have port-channel working.