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6.16
UDLD
The UDLD (UniDirectional Link Detection) protocol is used to detect and/or disable
unidirectional connections before they create dangerous situations such as Spanning Tree loops
or other protocol malfunctions.
The UDLD protocol was implemented to help correct certain assumptions made by other
protocols and in particular to help the STP to function properly so as to avoid the creation of
dangerous Layer 2 loops.
UDLD is meant to be a Layer 2 detection protocol that works on top of existing Layer 1
detection mechanisms defined by the IEEE standards.
Network systems typically check for miss wiring or unidirectional problems at the physical
layer. The physical layer defines the actual physical components such as connectors and cables.
So, physical layer only checks for open lines or port malfunctions and would not detect a miss
wired systems.
But, UDLD performs mutual neighbor identification; in addition, it performs neighbor
acknowledgement on top of the Logical Link Control (LLC) layer and thus is able to discover
logical one-way miscommunication between neighbors even when either one of PHY layer
mechanisms has deemed the transmission medium bidirectional.
6.16.1
Port Settings
6.16.1.1
CLI Configurations
Node
Command
Description
enable
show udld status
This command displays the UDLD global settings.
enable
show udld port-status This command displays the ports’ settings.
enable
show udld neighbor
This command displays the port’s neighbor
information.
enable
configure terminal
This command changes the node to configure node.
config
udld enable
This command enables the global UDLD state.
config
no udld enable
This command disables the global UDLD state.
config
udld message interval-
time <1-90>
This command configures the interval time of
sending . (unit: second).
config
udld message interval-
time reset
This command configures the interval time to
default value. (7 seconds)
configure interface IFNAME
This command enters the interface configure node.
interface
udld port enable
This command enables the udld state for the specific
port.
interface
no udld port enable
This command disables the udld state for the
specific port.
interface
udld port aggressive
This command configures the udld mode to
aggressive mode for the specific port.
interface
no udld port
aggressive
This command configures the udld mode to normal
mode for the specific port.
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