It has been a while since I didn’t post anything here, but it was holidays and I used that time to relax and rest after a year of work. Following this idea I wish you all “Happy New Year” and all the best in 2009.

Today I planned to write about something easy to implement (just to get in shape), but ignored by some network engineer. For me, Layer 1 issues are very annoying, and here I’m talking mostly about the cases when everything look perfect on your side, cable is plugged in, you have green light for the link, but nothing is working.

Luckily some smart engineers think to develop and implement a feature called Unidirectional Link Detection (UDLD). UDLD is used to detect when the send channel (Tx) of a cable is down, but not the receive channel (Rx) and vice versa. This situation typically can occur in a fiber optic cable when there is a break on one side of the cable run or in copper cable when Rx or Tx pair is broken. When UDLD detects this situation the interface is brought down to prevent spanning-tree loops and black holes due to  unidirectional links.Remember, UDLD is a Layer 2 protocol that with Layer 1 mechanisms to determine the physical status of a link.

Please have a look below for a configuration example:

UDLD

Cisco: Layer 1 link failure detection
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