Administrator's Guide v4.1.1 Manual

Configuring UDLD
● UDLD overview............................................................................................................. 433
● Configuring UDLD......................................................................................................... 435
● Other UDLD-related commands....................................................................................435
UDLD overview
The UniDirectional Link Detection (UDLD) protocol is a nonstandard Layer 2 protocol that detects when
a physical link becomes unidirectional by means of the exchange of UDLD protocol data units (PDUs).
A unidirectional loop can lead to the creation of a loop in a network, which the Spanning Tree Protocol
(STP) could inadvertently allow to occur.
UDLD requirements
Note the following requirements for UniDirectional Link Detection:
• Network OS 4.0 or later.
• UDLD runs only on physical ports assigned to a port channel.
• UDLD is supported on directly connected switches only.
• UDLD works in standalone mode or VCS mode. In VCS mode, UDLD applies only to edge ports.
• UDLD can interoperate with Brocade IP products.
How UDLD works
The following shows a simple four-switch network in which two paths connect to each switch. STP
blocks traffic on as many ports as necessary so that only one operational path exists from the STP root
bridge to all nodes in the network.
Network OS Administrator’s Guide
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