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Cisco Small Business 200, 300 and 500 Series Managed Switch Administration Guide (Internal Version) 146
Port Management: Unidirectional Link
Detection
This section describes how the Unidirectional Link Detection (UDLD) feature.
It covers the following topics:
• UDLD Overview
• UDLD Operation
• Usage Guidelines
• Dependencies On Other Features
• Default Settings and Configuration
• Before You Start
• Common UDLD Tasks
• Configuring UDLD
UDLD Overview
UDLD is a Layer 2-protocol that enables devices connected through fiber-optic or
twisted-pair Ethernet cables to detect unidirectional links. A unidirectional link
occurs whenever traffic from a neighboring device is received by the local device,
but traffic from the local device is not received by the neighbor.
The purpose of UDLD is to detect ports on which the neighbor does not receive
traffic from the local device (unidirectional link) and to shut down those ports.