http://gns3vault.com UDLD (Unidirectional Link Failure) can be used to prevent layer 2 loops on fibre links in case either the transmit or receive link fails. UDLD will send a "hello" packet to the other side, if this fails then the link will be shut to prevent a loop. This video explains you how to configure UDLD and demonstrates how it prevents a layer 2 loop.
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