Unidirectional Link Detection

<p>Unidirectional Link Detection (UDLD) is a <a href="/facts/Data_link_layer/K9yPufbt">data link layer</a> protocol from <a href="/facts/Cisco_Systems/ocDgQt6T">Cisco Systems</a> to monitor the physical configuration of cables and detect unidirectional links.  UDLD complements the <a href="/facts/Spanning_Tree_Protocol/ofHHMBuM">Spanning Tree Protocol</a> which is used to eliminate <a href="/facts/Switching_loop/QtN2c9St">switching loops</a>.
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<ul><li>Unidirectional Link Detection (UDLD) is one of two major features (UDLD and loop guard) in Cisco Switches to prevent Layer 2 loops.</li>
<li>Spanning-Tree Protocol (STP) resolves redundant physical topology into a loop-free, tree-like forwarding topology via blocking one or more ports.</li></ul>
However,  Unidirectional Link failure can cause "<a href="/facts/Black_hole_(networking)/EB12lgNj">traffic blackholing</a>" and loops in the Switch topology.
<ul><li>In order to detect the unidirectional links before the forwarding loop is created, UDLD works by exchanging protocol packets between the neighboring devices.</li>
<li>In order for UDLD to work,  both switch devices on the link must support UDLD and have it enabled on respective ports.</li></ul>

Unidirectional Link Detection open-in-new

Unidirectional Link Detection