Ethernet Cross Over Cables between Servers

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Oct 21st, 2008
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When, if ever, is it OK to use a ethernet cross over cable between two server clusters in a production environment? What are the benefits of using a switch?

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Joseph W. Doherty Wed, 10/22/2008 - 04:11
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If there's only communication between two hosts, no real advantage of a switch except perhaps as another way to capture traffic stats.

If you want to communicate between more than two hosts, something beyond a single cross over cable makes that a bit easier.

vjlsmalls1 Wed, 10/22/2008 - 09:17
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Thanks. That's how I feel. However, some people think that a switch somehow increases or propogates the electrical signal between the two boxes,, like a repeater. Thoughts? I think that adding a switch adds an additional and unecessary point of failure. Is there any industry best practice documentation that you could refer me to?

Joseph W. Doherty Wed, 10/22/2008 - 10:04
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Actually switches do rebuild frames, so you can extend the distance beyond end hosts. But besides adding a point of failure, you also need to insure the switch can actually handle the offered traffic load and also note the switch will add some latency.

vjlsmalls1 Wed, 10/22/2008 - 14:32
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We had a server on a 3550-48 XL switch. The interface was rapidly accumulating thousands of underruns causing the cluster to fail over. We moved the connections to another 3550 and it still acumulated the underruns. So we put a cross over cable between the devices and the cluster stopped dropping frames and failing over. The cluster is performing well.

We had another server on a 3550 that was doing the same thing and we put it on a 4507 and it stopped. Where can I find information about the backplane fabric capacities for switches? Thanks for all of your help.

glen.grant Wed, 10/22/2008 - 14:42
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When you are having a problem with the overrruns use the "show controllers utilization " command and see if the port or switch fabric is being overwhelmed. Stats for the 3550 are here. For any switch just do a search on the main cisco page for data sheets, such as 4500 data sheets or 3750 data sheets.


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