Cisco 350 Series Administration Manual page 226

Port Management
UDLD
Cisco 350, 350X and 550X Series Managed Switches, Firmware Release 2.4, ver 0.4
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.
• 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:
- Fiber interfaces are in the global UDLD state.
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