Cisco 3560 to 3750 MMF why would a SH GBIC not work but a LH GBIC work?
Posted on 2007-11-14
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.