Load Balancing- Etherchannel

Unanswered Question
Jun 16th, 2009

Hi, I have one query regarding Load balancing in Etherchannel. I have many internal client which is reaching ONE server. My load balance should be on Source-Mac or Dst-Mac ? When we can use Sourcr-Mac? please help.

I have this problem too.
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Giuseppe Larosa Wed, 06/17/2009 - 00:03

Hello Rupesh,

the exact behaviour is platform dependent more recent platforms are able to detect the traffic type and to use a different algorithm accordingly

you can check with

sh etherchannel load-balance

EtherChannel Load-Balancing Configuration:

src-dst-ip

EtherChannel Load-Balancing Addresses Used Per-Protocol:

Non-IP: Source XOR Destination MAC address

IPv4: Source XOR Destination IP address

IPv6: Source XOR Destination IP addres

this says ipv4 SA exor ipv4 DA that is fine for your scenario.

This was taken from a C4506 with cat4500-ipbasek9-mz.122-50.SG1.bin.

Hope to help

Giuseppe

Rupesh Kashyap Wed, 06/17/2009 - 01:35

I know the configuration part. I am only confused when to use Dst-Mac based and when to use Source-mac based??

Joseph W. Doherty Wed, 06/17/2009 - 03:24

With Etherchannel, you want to "mix" the Etherchannel hash attribute(s) to try to obtain the best usage of your multiple channels.

Assume you have a router Etherchannelled to a switch which has multiple hosts connected, and those hosts use the router as the gateway.

All of subnet traffic going off subnet, from the multiple hosts, will be go to the gateway's interface single MAC although come from different MACs. All traffic entering the subnet from gateway interface will be sourced from the gateway's single MAC yet likely be going to different hosts, each with different MACs.

Given just the options dst-mac and src-mac to hash on, we want to use dst-mac for traffic from the gateway to hosts, since the src-mac wouldn't change. Conversely, traffic from hosts to the gateway, we want to use src-mac since the dst-mac wouldn't change.

Giuseppe Larosa Wed, 06/17/2009 - 04:40

Hello Rupesh,

I tried to show you that switches can be so smart to use IPv4 parameters for load balancing of ipv4 traffic over an etherchannel:

IPv4: Source XOR Destination IP address

on a modern switch the MAC addresses are used for non IPv4 traffic as shown in my previous post.

Hope to help

Giuseppe

Giuseppe Larosa Wed, 06/17/2009 - 08:50

Hello Rupesh,

being by default the Exor of IP SA and IP DA you should get a fair balancing in your case in either direction

the exor involves the less significant bits so that an index is calculated and gives the pointer to a member link.

if the etherchannel is made of 2 links is enough one bit if 4 links by using two bits and so on.

if the number of links is odd like 3 3 bits are used and the values are associated to links (3 3 2)

Hope to help

Giuseppe

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