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Cabling Question

Posted on 2015-01-07
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Last Modified: 2015-01-12
I have what is most likely a very simple question. Is the cable that connects a Juniper SG8 to a Cisco 48 port managed switch a crossover cable?
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Question by:Dennis Pillow
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by:Don Johnston
ID: 40536666
I couldn't find a Juniper SG8.  And you didn't say which Cisco switch.  So it's hard to say.  

That said, most current Cisco switches support Auto-MDIX which means it will automatically adapt to whatever the other device is no matter what cable you use.
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by:Don S.
ID: 40536668
That depends on the exact models involved.  Generally speaking, they stopped making fixed polarity switches quite a while ago, although some are still in use.  With most switches these days supporting auto MDI/MIDX, it doesn't matter any more - usually.
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by:Dennis Pillow
ID: 40536867
I will get the exact model number on the switch from my customer. This was a question from them.
I will post an update tomorrow.
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by:Fred Marshall
ID: 40536985
The answer to the exact question is "maybe".  They asked about the cable and not the boxes, right?

"It may be a crossover cable and it may not be a crossover cable and it likely doesn't matter".

If it's already there and it's working then, again, it may not matter which wiring configuration that it has.

Consider this:

- if it's a straight-through cable then you won't be needing a crossover cable between those ports in the future unless you change one of the boxes and put in a really old box as a replacement.
- if it's a crossover cable then you may need a crossover cable between those ports in the future unless you change one of the boxes for a newer one.
- if it's a crossover cable then you may NOT actually need a crossover cable between those ports now or in the future.

There's a rather high probability that it's a straight-through cable.  It's easy to tell.  If it is then you're likely done.

If it's a crossover cable then you will probably want to know about the devices port capabilities - to determine if it can be replace with a straight-through cable immediately.  If it can be then that's what I'd do.
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by:Dennis Pillow
ID: 40537053
Thank you fmarshall I will check on the switch number tomorrow and see what it is.
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skipskip earned 500 total points
ID: 40543805
Another thing to keep in mind is if you utilize a straight-through cable and allow auto mdix to do the cross over, you cant force/hard code the speed or duplex. By hard coding the speed and duplex you are essentially disabling the L1 link negotiation that includes mdix. Typically if you are connecting switches, router, servers or firewalls its good practice to  hard code the speed and duplex .
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Author Closing Comment

by:Dennis Pillow
ID: 40544570
Great advice, thank you.
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