3750X Load Balancing Hash

[atlantas0n]

Hello community,

I have a Cisco 3750X switch that has a port-channel consisting of two links to a downstream Nexus 5548.   In this port-channel are interfaces Gi2/1/1 and Gi2/1/3.  I have noticed that approximately 400 packets/s are being forwarded out of Gi2/1/3 while only 2 packets/s are being forwarded out of Gi2/1/1.   After looking closely, I found out the my load balancing method was set at src-mac.   I switched to src-dst-mac and noticed some improvement but still not the even distribution I was looking for.   Upstream from this switch is one physical connection to my router/default gateway.  My question is this:  For a 3750X, If i have the source mac and destination mac, is there a way to manually figure out which link in the port-channel traffic will be forwarded out of?  For example, using load balancing method src-dst-mac, and my source MAC is aaaa.aaaa.aaaa and my destination MAC is bbbb.bbbb.bbbb, is there a way to manually determine whether traffic will use Gi2/1/1 or Gi2/1/3?  I know about the "test etherchannel" command.  But i want to see if I can manually determine before running the command for verification.

[Reza Sharifi]

Hi,

As far as I know, it is all based on the hashing algorithm and there is no way to manually send some traffic to one physical interface and some to the other.

HTH

[atlantas0n]

Hi Reza

Right, i'm trying to figure out the hashing method.   I'm not trying to manually send traffic down a particular port-channel interface, just trying to understand the switch is hashing the bits so that I can understand which link it will choose

[Joseph W. Doherty]

Disclaimer

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Posting

Unfortunately, not an answer to your question, but just wondering whether you have tried hashing using src-dst-IP, which I think the 3750X supports.

[atlantas0n]

Hi Joseph

I haven't tried that yet but it is my next step.  I just wanted to see if I there was a way to figure out the the src-dst-mac hash so that I could understand the traffic flows based off of a known source MAC and known destination MAC.   I've seen some writings where it shows how the 6500 series hashes.  It basically takes the last bit in the source MAC, the last bit in the destination MAC, and does an XOR on the two values. 

For example, lets use a traffic flow with source MAC aaaa.aaaa.aaaa and with two destination MACs-  bbbb.,bbbb.bbbb and eeee.eeee.eeee on a two-link port-channel. You take the last bit from the source and destination MACs and convert them to binary.  The last bit in the source mac after converting to binary would be 0.  The last bit in the destination MAC ending in "b" would be 1, while the last bit in the destination MAC ending in "e" would be 0.  

Traffic Flow #1

a = 10, b = 11

10 in binary is 00001010... last bit is 0

11 in binary is 00001011... last bit is 1

after XOR, the result is = 1 (traffic goes down 2nd segment)

Traffic Flow #2

a = 10, e = 14

10 in binary is 00001010... last bit is 0

14 in binary is 00001110... last bit is 0

after XOR, the result is = 0 (traffic goes down 1st segment)

At least for 6500 series, using this method, I should be able to figure out which link will be used if I have a known source MAC and destination MAC.  The problem is, the Cisco 3750X that I have is not following this hashing algorithm.  This is why I am wondering if anyone knows the method a Cisco 3750 will use to determine which link to use.

Source: Cisco LAN Switching (CCIE Professional Development, pg. 311, by Kennedy Clark)

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Realize exactly how Cisco equipment hashes is likely considered proprietary information, and also might be changed between IOS versions.