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fibre connectivity problems with cat2950 switches

Hello all,

we currently have 2 3com switches connected over mmf fibre for a distance of approx 1km, my problem is:

I have purchased 2 * 2950 switches and 2 * GBIC gigabit fibre module cards (multi mode). I am trying to swap out the current 3com installation. I understand that for the distance involved I will not be able to run gigabit and instead will have to run the cards at 100mbs. When I directly connect the switches a link light shows and traffic can be passed. However when I try to connect over the fibre run I can get no connection light at all.

Can someone advise on the config changes i need for each gbic port and if there is a problem with the distance involved how would i identify this from the output of the ciscos ?

I have checked the fibre run itself and all is ok there..

many thanks



Re: fibre connectivity problems with cat2950 switches

GBICs are GigaBit Interface Converters, they do not downshift to 100-megabit Fast Ethernet speed. They are Gigabit Ethernet only.

SX GBICs require multimode fiber only; but are limited to a distance of 220m to 275m, depending on the type of fiber installed.

To run over multimode fiber out to distances of 550m, you can use the more expensive LH/LH GBICs. But you also need special mode conditioning launch or patch cables.

For distances beyond 550m, at Gigabit Ethernet speeds, you need single mode fiber.

That being said, I remember reading about how some vendors pushed GbE over MMF well past the specified distances at one of the trade shows (might have been NetWorld/Interop, many years ago). But I don't think you're going to have any luck if your distance is truly 1km of MMF.

When you tried to connect over the 1km of fiber, did you try swapping the connectors on the GBIC at one end of the link? Because if you tested locally with a crossover patch and it worked, then used the same kinds of patches at each end of the longer fiber run, you may have crossed your crossover, turning it into a straight-through connection which would also give you no link.

I have several customers who put in MMF years ago, buried it when FDDI and ATM OC-3 networking was hot. They were using 3Com equipment, too (LANplex, CELLplex, and CoreBuilder boxes). They were good to 2km point-to-point. Fast Ethernet over fiber, 100BASE-FX, is also good to 2km over MMF when run in full duplex (only 412m in half duplex). When Gigabit Ethernet came out, these customers found they couldn't run it over their MMF, the distances between buildings was too far. So instead, they're running multiple 100-megabit connections using Fast Ethernet media converters from Transition Networks, and using Fast EtherChannel on the Cisco switches, to get more bandwidth for now. This was less expensive than running new single mode fiber for them; but they know that the long-term solution is to install new SMF.

Hope this helps.