This is probably more than you wanted to know, but...
If it worked, I'm sure it was across a very short distance (like putting two patch cords together). If you got any distance out of that, you should do it only for a test, not for production. To see if any interface is having problems (GBICs as well as any other interface), do a SH INTERFACE and look at the error counters.
The correct way to go from Multimode (MM) to Single Mode (SM) is to use a "Mode Conditioning" patch cord. However, people almost exclusively want to drive a long stretch of MM fiber using a SM device (a GBIC in this case) to drive it longer distances than a MM device could. I don't even know if there's products to put MM electronics onto SM fiber...
Here's my "operational" explanation of the difference between SM and MM fiber, without all the optical physics B.S. that I don't understand anyway...
MM fiber is less expensive glass to produce than SM, and has less expensive electronics (MM devices usually use cheaper LEDs instead of lasers). The MM glass itself is made to transmit light that enters at different angles (the light is "blurred"). The cable is also alot thicker than SM (62.5 or 50 microns). The light also reflects off the sides of the fiber pipe as it transverses the cable. In order to get that reflectivity, the outside perimeter is actually clearer than the core of the glass. MM is mainly used for in-building and campus cabling.
If you want to see how it reflects the angled light off the sides, look into an aquarium from one end sometime. Notice how the sides are reflective like a mirror at different angles? That's how the sides of the MM glass looks from the inside. A big shiney pipe.
SM is more expensive to install, has more expensive electronics, but goes alot longer distances. SM glass is made to propogate a narrowly focused beam of light right down the center of the fiber (the fiber is only 9 microns thick), and doesn't tolerate light introduced at angles. If the light beam isn't launched into the cable perfectly straight, it won't go far. Lasers produce light, by definition, as a narrowly focused beam. The electronics are built to much more strict tolerances, thus raising their cost.
The bottom line is that MM cable is cheaper to install, has less expensive electronics, but doesn't drive as far as SM. By the way, GigE doesn't go nearly as far as 100Base-FX. SM, on the other hand, is used for inter-campus, and long distance applications.
After knowing how MM fiber has a "blurry" core and a clear perimeter, you will be interested to know that the "Mode Conditioning" patch cord takes the narrow beam of a SM laser and offsets it to travel down the side of MM fiber. Normally, the SM laser launches the light right down the middle of the fiber. In order to take a MM device and launch it into a SM fiber, you'd need to focus the blurry beam. The reason it works for short distances is that the small amount of light that happened to go straight into the SM fiber was enough to get to the other end of the cable, before it attenuated. The light would be too dim to go very far, since almost all of it is angled.
Finally, if you get into pulling fiber, pick a fiber cabling vendor (but not a contractor who installs it) who knows fiber intimately, and spend time selecting the proper cable and getting it designed and installed professionally. The cabling itself is often more expensive than the electronics driving it. Since the life of cabling is alot longer than the life of electronics, it pays to design cabling that can survive several generations of electronic applications.
Another long one. I hope it is all useful to someone out there!
This thing could happen not only in some "patch cord" cases, but also because your switch has a GBIC support both MMF and SMF.
As we know, Cisco now has 3 models of GBIC module: WS-G5484, WS-G5486 and WS-G5487. Data from Official Cisco documents says a G5484 SX GBIC supprots 850nm wavelenth MMF tansfer traffic in distance from 722ft to 1804ft as its core size and modal bandwidth varies. The G5487 ZX GBIC can load traffic upto a distance of 62mile using 1550nm 9/10micron SMF.
And the 5486 LX/LH GBIC can create a link up to 1804ft using 1300nm 50.0micron MMF, while the distance could also be up to 10 kilometer by using 9/10 micron core size SMF. So, if you have installed a WS-G5486 GBIC in the GBIC slot on your switch, it will support both MMF and SMF. Only availale cable distance will be different.
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