Usage Guidelines; Dependencies On Other Features - Cisco Small Business 200 Series Administration Manual

Port Management: Unidirectional Link Detection

Usage Guidelines

Reactivating a Shutdown Port
You can reactivate a port that was shut down by UDLD in one of the following ways:
• Automatically—You can configure the system to automatically reactivate ports shut down by UDLD
in the Port Management > Error Recovery Settings page. In this case, when a port is shut down by
UDLD, it is automatically reactivated when the automatic recovery interval expires. UDLD again
begins running on the port. If the link is still unidirectional, UDLD shuts it down again after the UDLD
expiration time expires, for instance.
• Manually—You can reactivate a port in the Port Management > Error Recovery Settings page
Usage Guidelines
Cisco does not recommend enabling UDLD on ports that are connected to devices on which UDLD is not
supported or disabled. Sending UDLD packets on a port connected to a device that does not support UDLD
causes more traffic on the port without providing benefits.
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.
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