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Switch:  Combined Ports

Posted on 2007-04-01
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Last Modified: 2010-04-12
Hello Experts,

I have a question in regards to linking two switches together.  Lets say I have two Catalyst switches, both are 3750's and I connected fa0/10, fa0/11and fa0/12 from SW1 to SW1 respectively.  Let say that each port is running at 100Mb.  Does this mean that the total link between the switches is 300 Mb?  If not, how can this be achieve via CAT5?  What commands do I need to read up?

As always, thank you in advance for your time on this matter.
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Question by:coperator
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mdrichards earned 250 total points
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Yep you certainly can, although with 3750's you'd wanna use the builtin backplane as it is much faster.

If you want to group standard ethernet ports together you want to set up an "EtherChannel"

You basically group fa0/10, 0/11 and 0/12 into a "channel-group" making them a virtual 300 Mb channel.

Check out the links below

http://en.wikipedia.org/wiki/EtherChannel

http://www.cisco.com/en/US/tech/tk389/tk213/technologies_configuration_example09186a0080094647.shtml
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by:Don Johnston
Don Johnston earned 250 total points
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>Does this mean that the total link between the switches is 300 Mb?

No. Spanning tree will block all but one link resulting in a single, operational 100mb link.

>If not, how can this be achieve via CAT5?

The closest you can come to doing that is etherchannel. However, Etherchannel groups must be 2, 4 or 8 links. If you try it with 3, the best outcome would be that only 2 of the links would carry traffic.

Even if you use a 4 port etherchannel group, it will still not give you a 400mb "pipe" between the switches. The traffic between two end devices (say A and B) that cross the etherchannel will use one of the 4 fastethernet links. The traffic between C and D may use a different link.

The bottom line is that etherchannel doesn't give you a faster path between two switches. It's really just a way of trying to get around spanning-tree blocking the redundant ports. At the end of the day, if you need more bandwidth, a faster link is the only way to go.
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