- Preface
- Overview
- Using the Command-Line Interface
- Configuring the Switch Alarms
- Performing Switch Setup Configuration
- Configuring Cisco IOS Configuration Engine
- Configuring Switch Clusters
- Performing Switch Administration
- Configuring PTP
- Configuring PROFINET
- Configuring CIP
- Configuring SDM Templates
- Configuring Switch-Based Authentication
- Configuring IEEE 802.1x Port-Based Authentication
- Configuring Web-Based Authentication
- Configuring Interface Characteristics
- Configuring Smartports Macros
- Configuring VLANs
- Configuring VTP
- Configuring Voice VLAN
- Configuring STP
- Configuring MSTP
- Configuring Optional Spanning-Tree Features
- Configuring Resilient Ethernet Protocol
- Configuring Flex Links and the MAC Address-Table Move Update Feature
- Configuring DHCP
- Configuring Dynamic ARP Inspection
- Configuring IP Source Guard
- Configuring IGMP Snooping and MVR
- Configuring Port-Based Traffic Control
- Configuring SPAN and RSPAN
- Configuring LLDP, LLDP-MED, and Wired Location Service
- Configuring CDP
- Configuring UDLD
- Configuring RMON
- Configuring System Message Logging
- Configuring SNMP
- Configuring Network Security with ACLs
- Configuring QoS
- Configuring Auto-QoS
- Configuring EtherChannels
- Configuring Static IP Unicast Routing
- Configuring IPv6 Host Functions
- Configuring Link State Tracking
- Configuring IPv6 MLD Snooping
- Configuring Cisco IOS IP SLAs Operations
- Configuring Layer 2 Network Address Translation
- Troubleshooting the Cisco IOS Software
- Working with the Cisco IOS File System, Configuration Files, and Software Images
Configuring UDLD
Finding Feature Information
Your software release may not support all the features documented in this chapter. For the latest feature information and caveats, see the release notes for your platform and software release.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Prerequisites for UDLD
Restrictions for UDLD
Information About UDLD
UDLD
UniDirectional Link Detection (UDLD) is a Layer 2 protocol that enables devices connected through fiber-optic or twisted-pair Ethernet cables to monitor the physical configuration of the cables and detect when a unidirectional link exists. All connected devices must support UDLD for the protocol to successfully identify and disable unidirectional links. When UDLD detects a unidirectional link, it disables the affected port and alerts you. Unidirectional links can cause a variety of problems, including spanning-tree topology loops.
Modes of Operation
UDLD supports two modes of operation: normal (the default) and aggressive. In normal mode, UDLD can detect unidirectional links due to misconnected ports on fiber-optic connections. In aggressive mode, UDLD can also detect unidirectional links due to one-way traffic on fiber-optic and twisted-pair links and to misconnected ports on fiber-optic links.
In normal and aggressive modes, UDLD works with the Layer 1 mechanisms to learn the physical status of a link. At Layer 1, autonegotiation takes care of physical signaling and fault detection. UDLD performs tasks that autonegotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected ports. When you enable both autonegotiation and UDLD, the Layer 1 and Layer 2 detections work together to prevent physical and logical unidirectional connections and the malfunctioning of other protocols.
A unidirectional link occurs whenever traffic sent by a local device is received by its neighbor but traffic from the neighbor is not received by the local device.
In normal mode, UDLD detects a unidirectional link when fiber strands in a fiber-optic port are misconnected and the Layer 1 mechanisms do not detect this misconnection. If the ports are connected correctly but the traffic is one way, UDLD does not detect the unidirectional link because the Layer 1 mechanism, which is supposed to detect this condition, does not do so. In this case, the logical link is considered undetermined, and UDLD does not disable the port.
When UDLD is in normal mode, if one of the fiber strands in a pair is disconnected, as long as autonegotiation is active, the link does not stay up because the Layer 1 mechanisms detects a physical problem with the link. In this case, UDLD does not take any action and the logical link is considered undetermined.
In aggressive mode, UDLD detects a unidirectional link by using the previous detection methods. UDLD in aggressive mode can also detect a unidirectional link on a point-to-point link on which no failure between the two devices is allowed. It can also detect a unidirectional link when one of these problems exists:
- On fiber-optic or twisted-pair links, one of the ports cannot send or receive traffic.
- On fiber-optic or twisted-pair links, one of the ports is down while the other is up.
- One of the fiber strands in the cable is disconnected.
In these cases, UDLD disables the affected port.
In a point-to-point link, UDLD hello packets can be considered as a heart beat whose presence guarantees the health of the link. Conversely, the loss of the heart beat means that the link must be shut down if it is not possible to reestablish a bidirectional link.
If both fiber strands in a cable are working normally from a Layer 1 perspective, UDLD in aggressive mode detects whether those fiber strands are connected correctly and whether traffic is flowing bidirectionally between the correct neighbors. This check cannot be performed by autonegotiation because autonegotiation operates at Layer 1.
Methods to Detect Unidirectional Links
UDLD operates by using two methods:
UDLD learns about other UDLD-capable neighbors by periodically sending a hello packet (also called an advertisement or probe) on every active port to keep each device informed about its neighbors.
When the switch receives a hello message, it caches the information until the age time (hold time or time-to-live) expires. If the switch receives a new hello message before an older cache entry ages, the switch replaces the older entry with the new one.
Whenever a port is disabled and UDLD is running, whenever UDLD is disabled on a port, or whenever the switch is reset, UDLD clears all existing cache entries for the ports affected by the configuration change. UDLD sends at least one message to inform the neighbors to flush the part of their caches affected by the status change. The message is intended to keep the caches synchronized.
UDLD relies on echoing as its detection mechanism. Whenever a UDLD device learns about a new neighbor or receives a resynchronization request from an out-of-sync neighbor, it restarts the detection window on its side of the connection and sends echo messages in reply. Because this behavior is the same on all UDLD neighbors, the sender of the echoes expects to receive an echo in reply.
If the detection window ends and no valid reply message is received, the link might shut down, depending on the UDLD mode. When UDLD is in normal mode, the link might be considered undetermined and might not be shut down. When UDLD is in aggressive mode, the link is considered unidirectional, and the port is disabled.
If UDLD in normal mode is in the advertisement or in the detection phase and all the neighbor cache entries are aged out, UDLD restarts the link-up sequence to resynchronize with any potentially out-of-sync neighbors.
If you enable aggressive mode when all the neighbors of a port have aged out either in the advertisement or in the detection phase, UDLD restarts the link-up sequence to resynchronize with any potentially out-of-sync neighbor. UDLD shuts down the port if, after the fast train of messages, the link state is still undetermined.
Figure 33-1 UDLD Detection of a Unidirectional Link
Default UDLD Settings
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How to Configure UDLD
Enabling UDLD Globally
Follow these steps to enable UDLD in the aggressive or normal mode and to set the configurable message timer on all fiber-optic ports on the switch:
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udld { aggressive | enable | message time message-timer-interval } |
Specifies the UDLD mode of operation:
An individual interface configuration overrides the setting of the udld enable global configuration command. For more information about aggressive and normal modes, see the “Modes of Operation”.
Note This command affects fiber-optic ports only. Use the udld interface configuration command to enable UDLD on other port types. For more information, see the “Enabling UDLD on an Interface” section. |
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Enabling UDLD on an Interface
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Specifies the port to be enabled for UDLD, and enters interface configuration mode. |
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Note Use the no udld port interface configuration command to disable UDLD on a specified fiber-optic port. For more information about aggressive and normal modes, see the “Modes of Operation”. |
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Setting and Resetting UDLD Parameters
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(Optional) Enables the timer to automatically recover from the UDLD error-disabled state. |
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(Optional) Specifies the time to recover from the UDLD error-disabled state. |
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Maintaining and Monitoring UDLD
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Additional References
The following sections provide references related to switch administration:
Related Documents
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Standards
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No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature. |
MIBs
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To locate and download MIBs using Cisco IOS XR software, use the Cisco MIB Locator found at the following URL and choose a platform under the Cisco Access Products menu: http://cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml |
RFCs
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No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature. |