Port Management: Unidirectional Link Detection
Dependencies On Other Features
Cisco 500 Series Stackable Managed Switch Administration Guide 206
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In addition, take the following into consideration when configuring UDLD:
• Set the message time according to how urgent it is to shut down ports with
a unidirectional link. The lower the message time, the more UDLD packets
are sent and analyzed, but the sooner the port is shut down if the link is
unidirectional.
• If you want UDLD to be enabled on a copper port, you must enable it per
port. When you globally enable UDLD, it is only enabled on fiber ports.
• Set the UDLD mode to normal when you do not want to shut down ports
unless it is known for sure that the link is unidirectional.
• Set the UDLD mode to aggressive when you want both unidirectional and
bidirectional link loss.
Dependencies On Other Features
• UDLD and Layer 1.
When UDLD is enabled on a port, UDLD actively runs on that port while the
port is up. When the port is down, UDLD goes into UDLD shutdown state. In
this state, UDLD removes all learned neighbors. When the port is changed
from down to up, UDLD resumes actively running.
• UDLD and Layer 2 Protocols
UDLD runs on a port independently from other Layer 2 protocols running on
the same port, such as STP or LACP. For example, UDLD assigns the port a
status regardless of the STP status of the port or regardless of whether the
port belongs to a LAG or not.
Default Settings and Configuration
The following defaults exist for this feature:
• UDLD is disabled by default on all ports of the device.
• Default message time is 15 seconds.
• Default expiration time is 45 seconds (3 times the message time).
• Default port UDLD state:
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