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I have two Cisco 2950C switches with two trunk port connections between them. On one line the lights are green on both ends. On the other, the light turned amber only on one end (green on the other). The switch with the amber light also has two trunk connections to another switch. I was able to get the light to turn green by setting the port mode to trunk. But when setting it back to trunk desirable it turns amber again. I can see no other errors or problems, but I'm curios about this behavior.

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    (See Also: Spanning Tree) If you have two switches with two links between them, that's a loop. STP will detect this and shutdown ("block" or "discard") one of the paths. The amber LED is the visual indicator of this. – Ricky Dec 31 '14 at 21:16
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Ethernet is designed around a tree toplogy. Your switches support some variant of spanning tree protocol which has disabled forwarding on one of the ports to prevent a loop. As other answes have said the orange light indicates that forwarding is disabled.

If the switches did not support spanning tree you would have succeeded in creating a loop which would result in a broadcast storm and a broken network.

If you do want to use both links at the same time then you will need to configure some form of link aggregation.

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Fred this behavior would be due to Spanning Tree Protocol detecting the possible loop you've created.

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When a Port LED is Amber the Port is not forwarding. This is possibly due to the port being disabled by management, an address violation, or Spanning Tree Protocol (STP). After the port is reconfigured, the port LED can remain amber for up to 30 seconds while STP checks the switch for possible loops.

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2950/hardware/installation/guide/2950hig/hgovrev.htm

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