Etherchannel Load Balancing

cjthedj45

Hi,

I'm trying to figure out if there is a way to load balance traffic across port channel members evenly. We have a situation where a backup runs and causes one of the port channels to exceed its bandwidth. We have an ether channel with two Gb ports in between the Core and Server switch. The server switch has all servers patched in that need to be backed up. When the backup runs we receive alerts stating that one port channel is exceeding its bandwidth but no alerts for the other. I want to evenly distribute this traffic across both channels. This is not causing an issue as it is out of hours but as out network grows this could cause an issue.

The current configuration is this:
9 Po9(RU) - Gi1/0/26(P) Gi2/0/26(P)
sh etherchannel load-balance
EtherChannel Load-Balancing Configuration:
src-dst-ip

I don’t fully understand how the portchannel link is selected. You have a few options for configuration I.E
port-channel load-balance {dst-ip | dst-mac | src-dst-ip | src-dst-mac | src-ip | src-mac} global configuration command.

What I have read on Cisco leads me to believe that the address in the frame is hashed to a numerical value that then selects one of the links in the channel. My configuration is source and destination IP address for load balancing. Therefore does this mean that the IP address is hashed and given a value and then sent down the associated channel? If it does then I don’t understand why both channels don’t appear to be being utilized. As each hash should be different generating a different value each time. The only other explanation is that perhaps there is a certain server that has a really huge back up and for the whole duration of this back up the one port channel is used. If this is the case then does anyone know if this can be changed so that traffic does not consume the whole bandwidth of that channel but instead it used the other channel?

Coolhandluke

Hi,

Your right about the load balancing methods, when using Src & Dst addresses they are XOR'd first to produce a value for the last bit in the address (allowing a value of 0 or 1) which will choose the channel (port 0 in the channel or port 1 in the channel). If your destination backup server has a value of 1 for the last bit, then the only variable bit will be your source servers, so for each server that ends in a 1, channel 0 will be used (xor) and for others channel 1 will be used. If you workout your addresses on paper you can manually figure out which servers will use which paths. If you find that lots of the servers are using the same channel path, a better solution may be to select src-mac load-balancing as most data will be going from servers to a single destination and mac's should be a little more random.

hope this helps

CoolHandLuke