Etherchannel load balancing - which hosts use which link

[NGJ]

Hi.  Are there any switch commands that will detail which hosts use a particular link in an etherchannel.  i.e. on one of our 2960x switch stacks, 2 x 10Gb links are bundled in an etherchannel to a core switch.  The load balance algorithm used on the 2960x is source mac. From what I understand, once a host has used a particular link it will continue to use that link in the etherchannel (as long as the etherchannel remains intact).  Presumably this mac-to-link info is stored somewhere. So is it possible to view this to determine which hosts actually use which link in the etherchannel?  Thanks 

[brselzer · Accepted solution]

Hello,

The port-channel hashing algorithm is per flow not per host. Depending on your algorithm, you could have two flows from the same host using different links. You can check which algorithm you are using with this command

2960X#show etherchannel load-balance
EtherChannel Load-Balancing Configuration:
src-mac                                  <----Hashing based on source mac

EtherChannel Load-Balancing Addresses Used Per-Protocol:
Non-IP: Source MAC address
IPv4: Source MAC address
IPv6: Source MAC address

If you want to see which link a specific flow will use, you can use the following command

2960X#test etherchannel load-balance interface port-channel 1 mac aaaa.aaaa.aaa bbbb.bbbb.bbbb
Would select Gi2/0/1 of Po1 <----This will tell you what interface it will use

If you're hashing based on something other than source mac, you will need to change the test etherchannel command to provide the correct information (ie. source/dest ip)

Hope that helps!

[NGJ]

Hi Bradley. Thanks for the quick response and clarifying that different flows from the same host could use different links.  I was not aware of that.

I tried the 'test etherchannel load-balance......' command.  That's exactly what I was looking for.  It worked and will help diagnose some issue we are having.

Thanks