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This morning, I troubleshooted a fiber connection between two switches in the same building. It was a single-mode fiber. I knew this fiber was fine, because I had installed it myself and tested (with my OTDR) after installation. My customer was having an issue connecting two switches to each other using the fiber connection. We had six different strands to try it with going between these two rooms in this building.

After hours of troubleshooting and figuring out for certain (it took much persuading and proving) that it was NOT the physical fiber connection that was the problem. We determined it by bringing the one switch into the same room as the other switch and hooking them directly to each other.... no link light.

The one switch had a single-mode fiber port with a maximum connection speed of 100 Mbit/s... The other switch supported up to a 1000 Mbit/s. I would have thought that the 1000 Mbit/s switch would be able to automatically switch itself down to 100 Mbit/s, similar to how you can connect a 100BASE-T connection to a 10BASE-T connection with no issues. Is this normally the case? Is this normal? Do any fiber switches support the ability to automatically throttle between 100 Mbit/s and 1 Gbit/s? Is this a limitation of the switch we were using or is this not possible on any switch?

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up vote 18 down vote accepted

Gigabit Ethernet over fiber and 100Base-LX10 aren't compatible. Further, the gigabit switch will not "fall back" or negotiate speed over fiber. This behavior is perfectly normal for fiber-based Ethernet. While copper-based Ethernet typically does have speed and duplex negotiation capabilities, fiber-based Ethernet does not.

It sounds like you need to get a fiber media converter and attach it between the switches, to me. I'd put a low cost 100Base-LX10 to 100Base-TX media converter on the end with the gigabit switch and attach the media converter to a copper port on the gigabit switch. When the old 100Mbps switch is replaced with something that supports gigabit optics the media converter can be removed and the two switches can be connected directly to each other.

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Ok, thank you so much! Just wanted to confirm it! Funny, I never really work with 100Mbps Fiber. –  renosis Mar 12 '12 at 20:10
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