This chapter describes how to configure the UniDirectional Link Detection (UDLD ) protocol on the Catalyst 6500 series switches.
Understanding How UDLD Works
The UDLD protocol allows devices that are connected through the fiber-optic or copper (for example, Category 5 cabling) Ethernet cables to monitor the physical configuraion of the cables
and detect when a unidirectional link exists.When a unidirectional link is detected, UDLD shuts down the affected port and alerts the user.The unidirectional links can cause a variety of problems,including spanning-tree topology loops.
UDLD is a Layer 2 protocol that work with the Layer 1 mechanisms to determine the physical status of a link. At Layer 1 , autonegotiation takes care of physical signaling and fault detection. UDLD performs the tasks that autonegotiation cannot perform such as detecting the identities of neighbors and shutting down the misconnected ports.When you enable both autonegiation and UDLD , the Layer 1 and Layer 2 detections work together to prevent the physical and logical unidirectional connections and the malfunctioning of other protocols.
A unidirectional link occurs whenever the traffic that is transmitted by the local device over a link is received by the neighbor but the traffic that is transmitted from the neighbor is not received by the local device. If one of the fiber strans in a pair id disconnected , as long as autonegotiation is active, the link does not stay up .In this case, the logical link is undetermined , and UDLD does not take any action.If both fibers are working normally form a Layer 1 perspective, then UDLD at layer 2 determines whether those fibers are connected correctly and whether the traffic is flowing bidirectionally between the right neighbors .This check cannot be performed by autonegotiation ,because autonegotiation is a Layer 1 mechanism.
The switch periodically transmits the UDLD message (packets) to the neighbor devices on the ports that have UDLD enabled. If the messages are echoed back to the sender within a specific time frame and they are lacking a specific acknowledgment (echo), the link is flagged as unidirectional and port is shut down . The devices on both end of the link must support UDLD in order for the protocol to successfully identify and disable the unidirectional links.
Figure 31-1 shows an exapmle of a unidirectional link condition..Each switch can send packets to a neighbor swich but is not able to receive packets from the same switch that it is sending packets to. UDLD detects and disables these one-way connections.
Default UDLD Configuration
shows the default UDLD configuration
Feature Default Value
UDLD global enable state Globally disabled
UDLD per-port enable state for fiber optic media Enabled on all ethernet fiber-optic ports
UDLD per-port enable state for twisted-pair (copper) Disabled on Ethernet 10/100 and 1000
media BASE-TX ports
UDLD message interval 15seconds
UDLD aggressive mode disabled
Configuring UDLD on the Switch
Enabling UDLD Globally
To enable UDLD globally on the switch, perform this task in privileged mode:
set udld enable enable UDLD globally on the switch.
show udld verify the configuration
This example shows how to enable UDLD globally and verify the configuration:
consloe> (enable) set udld enable
UDLD enable globally
consloe> (enable) show udld
UDLD :enabled
console> (enable)
Enabling UDLD on Individual Ports
To enable UDLD on the individual ports, perform this task in privileged mode:
set udld enable mod/port Enable UDLD on a specific port
show udld prot mod/port verify the configuration
This example shows how to enable UDLD on port 4/1 and verify the configuration:
console>(enable) set udld enable 4/1
UDLD enable on port 4/1
console>(enable) show udld port 4/1
UDLD :enable
Message Interval :15 seconds
port admin status aggressive mode link state
4/1 enabled disabled bidirectional
console >( enable)
Disabling UDLD on Individual Ports
To disable UDLD on the individual ports , perform this task in privileged mode:
set udld disable mod/port disable UDLD on a specific port
set udld prot mod/port verify the configuration
This example shows how to disable UDLD on port 4/1:
console> (enable) set udld diable 4/1
UDLD disabled on port 4/1.
console>(enable)
Disabling UDLD Globally
To disable UDLD globally on the siwtch, perform this task in privileged mode:
set udld disable display UDLD globally on the switch
show udld verify the configuration
This example shows how to disable UDLD globally on the switch:
console> (enable) set udld disable
UDLD disabled globally
console>(enable)
Specifying the UDLD Message Interval
To Specify the UDLD message interval, perform this task in privileged mode:
set udld interval interval specify the UDLD message interval
show udld verify the configuration
This example shows how to secify the UDLD message interval on the switch:
console >(enable) set udld interval 20
UDLD message interval set to 20 seconds
console>(enable)
This example shows how to verify the message inverval on the switch:
console>(enable) show udld
Message Interval : 20 seconds
console>(enable)
Enabling UDLD Aggressive Mode
software release 5.4(3) and later releases have UDLD aggressive mode . UDLD aggressive mode is disabled by default and its use is recommened only for point-to-point links between the Cisco switches running software release 5.4(3) or later releases.With UDLD aggressive mode enabled, when a port on a bidirectional link that has a UDLD neighbor relationship established stops receiving the UDLD packets, UDLD tries to reestablish the connection with the neighbor. After eight failed retries, the port is put into errdisable state.
