Etherchannel Issue

Unanswered Question
Oct 30th, 2008

I have a 4506 and a 6509 with a 4 gig etherchannel between them. The interfaces are trunked. I am not able to get more than 1 gig of unidirectional throughput between them. I have tried Smartbits and iperf. Any suggestions as to the cause?

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Jon Marshall Thu, 10/30/2008 - 18:52

Frederick

Etherchannel uses load-balancing to utilise all the links. What it uses to load-balance depends on the switch model capabilities. See attached link for switch details -

http://www.cisco.com/en/US/tech/tk389/tk213/technologies_tech_note09186a0080094714.shtml

So for the same src/dst etherchannel will always use the same link. You need to

1) work out what type of etherchannel load-balancing your switches support - see link above

2) work out which load-balancing type of those available will give you the most variety in terms of src/dst.

Jon

glen.grant Fri, 10/31/2008 - 04:53

Jon pretty well explained it , it is a load sharing mechanism not a load aggregating mechanism . If you need speeds higher than 1 gig throughput you are going to have to go for the big bucks and have 10 gig interfaces on both ends of that link . As a 4 gig etherchannel I doubt you will ever fill that given that traffic has 4 different links to travel over.

frederick.w.moore Fri, 10/31/2008 - 06:39

Thank you both for the answers. I have a concern about a note in the link that Jon provided. It says:

"Note: Even if bundling four links forms the EtherChannel, the switch only uses two of them in order to transmit the data traffic. The other two links are kept for backup. The switch uses the lower order bits of source MAC address and destination MAC address in order to determine which links must be used to transmit the data. So if the data is received from the same source, then same link of the EtherChannel is used in order to forward the data."

I anticipate reaching 4 gig of traffic due to the 7 backup servers being connected to the 4506 switch. Currently there are 1500 hosts being backed up and the 1 gig uplink is getting saturated. The host count is expected to grow to about 4000 hosts. The backup time frame will not be increased.

Is it true then that I will only get 2 gig of unidirectional traffic because of the 4506 switch?

Jon Marshall Fri, 10/31/2008 - 09:08

Frederick

Yes that does seem to be what the note is suggesting. So if your backup servers are in different vlans you could then run multiple etherchannels with each etherchannel only a member of a specific vlan.

Other than this could you repatch some of the servers directly into the 6500 switch ?

Jon

frederick.w.moore Fri, 10/31/2008 - 09:16

All of the servers are in the same vlan. I am unclear on one concept. Does the etherchannel on the 4506 running IOS combine the individual bandwidths or is the etherchannel just for load balancing?

Rereading the document you sent the link for, it's would appear that CatOS on the 6500 will combine the bandwidths while IOS will only do load balancing? Is this correct?

I can't patch to the core due to the lack of fiber between locations. The 4506 servers is also hosting database servers, file servers, domain controller, etc. So the main concern is to gain as much bandwidth as possible without going to 10 gig.

Thanks again for the answers.

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