Real purpose for Etherchannel

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Oct 15th, 2008
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Can someone tell me what the real purpose is for an Etherchannel?


If I have 2 L2 switches with 3 ports bundled in an etherchannel, that allows for more bandwidth between switches, but if I have a 2950 that only has 100mb ports, then I don't really gain anything right?


Are they strictly for redundancy or are they load balancers too?


Thanks!

John

Correct Answer by Jon Marshall about 8 years 6 months ago

Ahh okay. You can run etherchannel to a end device such as a server and depending on load-balancing algorithms supported you could get increased bandwidth. But the server would need to be able to aggregate it's individual NIC's into an etherchannel as well.


Note confusingly most other hardware vendors i have come across call aN aggregation of ports a trunk link rather than an etherchannel. So when a Unix admin talks about a trunk they mean an etherchannel and not a link that can carry multiple vlans.


Generally though you are correct in that etherchannel is more commonly used between switches.


Jon

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VictorAKur Wed, 10/15/2008 - 05:54
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well... you will get 300 Mb of combined speed and good resilience. If one of the physical ports in the portchannel fails, you shouldn't even notice it (performance wise), unless you were maxing all 300 Mb at the time.

Jon Marshall Wed, 10/15/2008 - 06:32
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John


They do both. I'm not sure what you mean about a 2950 and only having 100Mb ports as you could still put a group of them into an etherchannel and get more throughput. Etherchannels do not need to be composed of gigabit links only.


Traffic is load-balanced across the individual links based on a combination of src/dst mac-address, src/dst IP address and TCP/UDP port numbers although different switches support only some of these methods.


They are also used for redundancy. The key thing about etherchannels is that they are seen by STP as only one link so you can have up to eight individual ports in the etherchannel group and still STP sees only one link.


Jon

John Blakley Wed, 10/15/2008 - 06:38
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Jon,


What I meant about the 2950 comment was that it makes no difference if I have 3 ports bundled into a 300mb etherchannel, I'm not getting any faster throughput to a host than 100mb. That being said, I know that the more links you have in etherchannel, the lower the cost on spanning-tree, and like you said, it sees it as one link and STP doesn't have to recalculate if a link goes down.


So, etherchannel is more of a bandwidth thing between switches, and it only helps with redundancy and load balancing for hosts. Is that a good way to look at it?


--John


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Jon Marshall Wed, 10/15/2008 - 06:44
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Ahh okay. You can run etherchannel to a end device such as a server and depending on load-balancing algorithms supported you could get increased bandwidth. But the server would need to be able to aggregate it's individual NIC's into an etherchannel as well.


Note confusingly most other hardware vendors i have come across call aN aggregation of ports a trunk link rather than an etherchannel. So when a Unix admin talks about a trunk they mean an etherchannel and not a link that can carry multiple vlans.


Generally though you are correct in that etherchannel is more commonly used between switches.


Jon

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