UDLD is a Cisco proprietary protocol used to detect and prevent unidirectional links in Ethernet networks
A unidirectional link is a situation where data can flow in only one direction and common causes being hardware failures such as a bad cable or port
UDLD helps prevent network problems that can occur when switches are not aware of the link failure potentially leading to network loops, Spanning Tree issues, or miscommunication between devices
How it Works:
To better understand UDLD, unidirectional links occur when data can only travel in one direction (either from switch A to switch B or vice versa), causing network issues such as loops, miscommunication, and instability
UDLD detects unidirectional links by periodically sending UDLD messages between directly connected switches
If a switch does not receive a UDLD response from the neighbor switch within a certain period, it considers the link to be unidirectional causing UDLD to take action
UDLD operates in two modes:
Normal Mode: UDLD detects and alerts about unidirectional links but does not take corrective action by itself. The administrator must manually intervene
Aggressive Mode: If UDLD detects a unidirectional link, it will automatically shut down the interface port after a certain number of failed messages
When to use:
UDLD should be implemented on fiber links as they are more prone to unidirectional issues. This type of implementation allows for a pro-active approach to detect faulty fiber links
UDLD should also be implemented on links that connect to core and backbone switches as these links are critical for network communication for downstream devices and clients at the access layer