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Cisco 3560 to 3750 MMF why would a SH GBIC not work but a LH GBIC work?

Posted on 2007-11-14
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Last Modified: 2013-11-09
Background:  I had a Black Box 100FX to 10MB TX transceiver connected to a Black Box switch that had a 100FX fiber input and 8 10MB ports.  The transceiver's 10MB port plugged into a Cisco 3750 ethernet port.  The 100FX from the transceiver went into a patch panel in the same cabinet which connected to another network cabinet where it stopped in another patch panel and then was patched through that same patch panel to another network cabinet where it stopped at the patch panel and then went to the Black Box 100fx fiber input.  This has worked fine for years.

So I wanted to upgrade this piece of the network for a project.  I purchased a Cisco 3560- 8 port POE switch and replaced the Black Box switch.  I purchased 2 GLC-SX-MM gbics.  I placed one of them into the 3560 and one into a spare gigabit port in the 3750.  These should have just worked over the muiltimode fiber.  Once I had replaced both ends it didn't work.  The symptoms were that the 3560 thought it was connected, link light on, and trunking on the gig port however couldn't see the switch via cdp neighbors command and lacked real connectivity.  The 3750 end didn't see any connection, had no link light, and seemed as if there was no switch plugged in at all.

I tried using other fiber pairs, replaced jumpers, went to the mid point where there was a passive jumper and replaced that with a new one as well.  No luck at all.  I had tried the switches locally via a LC-LC cable and they worked fine so I know the lc gbic were fine and the switches themselves were fine.  The fiber is the same fiber that has been in use for years.  The only difference was the speed I was trying to run over the line from 100MB to 1000MB.

I also went to the passive patch and brought a switch and connected it to both ends individually to make sure each individual switch on either side was functioning correctly over the fibers.

I remember reading that you can use Long Haul GBICS(LX) over Multimode fiber however I believe it still had the same distance limitation and really shouldn't make a difference other than it would work the same as a Short Haul GBIC.  I tried it and it worked no issues....WHY?

If anyone knows I would really like to understand why this works and why it happens.  I have read articles on Cisco and other books, etc...that state you can use Long Haul over Multimode but never that it would somehow work better.   If you need more details or don't understand what I explained feel free to ask.

Thanks!!!
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Question by:dis1931
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by:pseudocyber
ID: 20288084
What is the fine print/giberish on your fiber?
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by:dis1931
ID: 20292800
I'm afraid I don't understand the question.  Do you mean if there is any writing on the multimode fiber that is run between the cabinets?  If so let me know and I can take a look...and what should i be looking for numbers, words, etc...

Dis
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by:TreyH
ID: 20295941
I might have missed it, but what is the fiber length between the devices?
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by:jsbush
ID: 20306228
if you run a 'sho log' command on the fiber switches do you see anything amiss that references the GigabitEthernet ports?  if the network can allow it, run a 'clear log' first, then reset the switches, wait about 10 minutes or so, then run a 'sho log'.

As far as why it happens, I couldn't tell you without knowing what the exact problem the switches are having
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by:dis1931
ID: 20309094
TrevH,

I don't know the distance...maybe 600-900ft...just a guess.

jsbush,

While I didn't mention it...I did do a show log and no errors related to anything.  There are no issues with the gigabit port.  I also can not see the switch using show cdp ne.

However, to all like I said it wouldn't work with the SX module but worked perfectly fine with the LX module.  It is MM Fiber so why is this the case.  Does LX provide some longer/stronger signal that the SX over multimode?  An explanation.  Like I said this worked with the blackbox equipment and works fine with the new equipment if I use LX modules but not with the SX modules.  So there is no hardware or configuration issues.  I am really just looking for the technical reason why this would work.

Thanks
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TreyH earned 500 total points
ID: 20310978
Standard distance for 1000Base-SX with 62.5/125 multi-mode is 250 meters (~800ft) so you could be close to the limit. 100BASE-SX standard distance is 300 meters (980ft) which might be why the previous setup worked fine. 1000Base-LX standard is 550 meters over multi-mode. If the LX is running over 300 meters (over multi-mode), 'mode conditioning cables' should be used. Here's a little info about mode conditioning.

http://www.lanshack.com/ModeConditioning.aspx
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by:dis1931
ID: 20313517
TreyH,

I think you are right.  I had actually found a similar answer on Cisco's website but the gap was that I didn't know the distance for 100Base-SX nor had I thought to look.  It makes sense that it may have worked before and that it might be at the fringe if not past the 800ft mark.  Actually in talking to an employee that was hear when they ran those lines....he believes that at the time the distance limitations they had decided on were somewhere between 900-1000ft so it is likely that it is in that range.  I am awarding all the points to you and posting the link to the Cisco website that had similar information about the distance of SX/LX modules over MM/SM...

Thanks for filling in the gap for me.  This will certainly help me troubleshoot and understand some of the issues I have ran into in the past with similar situations.

http://www.cisco.com/univercd/cc/td/doc/product/core/7200vx/72vxfru/5067g.htm

Dis
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