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Purpose of Etherchannel

Posted on 2014-10-12
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Last Modified: 2014-10-17
If I understand When bundling a bunch of links to one Etherchannel, this will create fault tolerance to end hosts in case if one port fails, but there is no increase in bandwidth..
I have heard about Bandwidth increase and Load balancing benefits when using Etherchannel... but I do not see how does that happen.

Any clarification on that will be very appreciated

Thanks
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Question by:jskfan
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by:Don Johnston
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No, spanning tree provides fault tolerance with no increase in bandwidth.

Etherchannel will provide fault tolerance and some improvement in performance since all links are forwarding.
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by:Predrag Jovic
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Etherchannel will provide fault tolerance and some improvement in performance since all links are forwarding.

Just a little clarification of quote:

Bandwidth per link is the same, in can't be sum of all links.
Let's say you have 8 ports in channel, max theoretical bandwidth is 8Gb (if every link is 1Gb), but every host can use only 1 link at one time, so that host don't have more bandwidth then if there is just one link, but there is a algorithm that tries to evenly split hosts per links (I hope that this is understandable in English :) ). So, algorithm is more or less successful, but per host bandwidth most of time is greater than if etherchannel is not present. Theoretically in the best case scenario if you have 8 hosts and 8 links in channel, you can have 8 hosts and each have it's own dedicated link with 1Gb bandwidth. If one link fails host from that link would be transferred to some on other links etc ... I guess you got the point....
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by:jskfan
ID: 40375874
If we had 8 separate Trunk Links between 2 switches , would not be the same since each host can use only one Link.?
I agree We will loose fault tolerance , if one link fails the host cannot use other link.
But in matter of performance a host can use only one link any way.
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by:Predrag Jovic
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No, spanning tree provides fault tolerance with no increase in bandwidth.
Clarification of this quote :) is answer to your question:
You are forgetting about STP, only one link on L2 can be active at one time.  Other-ways you will have network loops.  Since trunking is happening at L2, the only way to have more bandwidth is bundling more ports in etherchannel.

8 separate Trunk Links between 2 switches
- means just one trunk active and seven blocked by STP :)
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by:Don Johnston
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We will loose fault tolerance , if one link fails the host cannot use other link.
No. With etherchannel, if one link fails, the traffic using that link will failover to one of the remaining links.
But in matter of performance a host cany use only one link any way.
Not exactly. A "flow" will use only one of the links. But a host can have multiple flows.
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by:jskfan
ID: 40375960
Correct ... I forgot about blocked ports.

the gray area about Etherchannel is the performance...I still cannot understand how this will provide performance, as long as the maximum bandwidth of each host is limited to the bandwidth of one link instead of all the Links in the group
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by:Don Johnston
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If you have 4 links in an etherchannel and 200 users, hopefully 25% of the traffic will be using one link, 25% using the other, etc.  

Which means that 50 of your users will be sharing one gig link (or whatever speed links you've got) instead of 200 users sharing a single gig.  Which is what would happen if you were not using etherchannel... Since spanning tree would block all links except one.
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by:jskfan
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OK the word Shared between hosts clears up.

so if 10 hosts sending 4GB of data at once through an etherchannel of 8GB(8 Links) then  the 4GB will be spread over 8 Etherchannel links and makes it to the destination faster than regular link.

Now if a single host send 4 GB of data will that be spread over all etherchannel links , and data makes it faster to the destination, than if it was just a single link of 1 GB ?
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Don Johnston earned 300 total points
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then  the 4GB will be spread over 8 Etherchannel links and makes it to the destination faster than regular link.
Hopefully.  But there is no assurance of balancing the traffic over the links. So some of the links could see more traffic than others.

Now if a single host send 4 GB of data will that be spread over all etherchannel links , and data makes it faster to the destination, than if it was just a single link of 1 GB ?
It depends on how the etherchannel load distribution is configured and whether the traffic is a single flow or multiple flows.

If the distribution is set to source IP, then the traffic will only use only one of the links.  

If the distribution is set to TCP/UDP port number AND there are multiple flows, then each flow could use different links.
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by:jskfan
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Thank you Guys !!
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by:Sean Hull CCIE 2052
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A couple of clarifications...

First the number of links that are used in an ether-channel are directly correlated to the hash algorithm that is used to spread traffic across links. For example the Cisco catalyst switch line uses a polynomial equation that is a combination of source destination MAC and IP address and now with Brocade Foundry/ Cisco Nexus switches use that and also includes the hash of the TCP/UDP session ID. Thus each TCP/UDP flow could use a different link.

Now spanning tree only applies to the port channel interface(the main etherchannel interface) not the individual interfaces in the ether channel. Of course spanning tree also applies to native interfaces that are not configured for Ether channel.
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