- Index
- Preface
- Product Overview
- Command-Line Interfaces
- Configuring the Switch for the First Time
- Administering the Switch
- Configuring Virtual Switching Systems
- Configuring the Cisco IOS In-Service Software Upgrade Process
- Configuring the Cisco IOS XE In Service Software Upgrade Process
- Configuring Interfaces
- Checking Port Status and Connectivity
- Configuring Supervisor Engine Redundancy Using RPR and SSO on Supervisor Engine 6-E and Supervisor Engine 6L-E
- Configuring Supervisor Engine Redundancy Using RPR and SSO on Supervisor Engine 7-E, Supervisor Engine 7L-E, and Supervisor Engine 8-E
- Configuring Cisco NSF with SSO Supervisor Engine Redundancy
- Environmental Monitoring and Power Management
- Configuring Power over Ethernet
- Configuring the Catalyst 4500 Series Switch with Cisco Network Assistant
- Configuring VLANs, VTP, and VMPS
- Configuring IP Unnumbered Interface
- Configuring Layer 2 Ethernet Interfaces
- Configuring EVC-Lite
- Configuring SmartPort Macros
- Configuring Cisco IOS Auto Smartport Macros
- Configuring STP and MST
- Configuring Flex Links and MAC Address-Table Move Update
- Configuring Resilient Ethernet Protocol
- Configuring Optional STP Features
- Configuring EtherChannel and Link State Tracking
- Configuring IGMP Snooping and Filtering, and MVR
- Configuring IPv6 Multicast Listener Discovery Snooping
- Configuring 802.1Q Tunneling, VLAN Mapping, and Layer 2 Protocol Tunneling
- Configuring Cisco Discovery Protocol
- Configuring LLDP, LLDP-MED, and Location Service
- Configuring UDLD
- Configuring Unidirectional Ethernet
- Configuring Layer 3 Interfaces
- Configuring Cisco Express Forwarding
- Configuring Unicast Reverse Path Forwarding
- Configuring IP Multicast
- Configuring ANCP Client
- Configuring Bidirectional Forwarding Detection
- Configuring Policy-Based Routing
- Configuring VRF-lite
- Configuring Quality of Service
- Configuring Voice Interfaces
- Configuring Private VLANs
- Configuring MACsec Encryption
- Configuring 802.1X Port-Based Authentication
- X.509v3 Certificates for SSH Authentication
- Configuring the PPPoE Intermediate Agent
- Configuring Web-Based Authentication
- Configuring Wired Guest Access
- Configuring Auto Identity
- Configuring Port Security
- Configuring Auto Security
- Configuring Control Plane Policing and Layer 2 Control Packet QoS
- Configuring Dynamic ARP Inspection
- Configuring DHCP Snooping, IP Source Guard, and IPSG for Static Hosts
- Configuring Network Security with ACLs
- Support for IPv6
- Port Unicast and Multicast Flood Blocking
- Configuring Storm Control
- Configuring SPAN and RSPAN
- Configuring ERSPAN
- Configuring Wireshark
- Configuring Enhanced Object Tracking
- Configuring System Message Logging
- Onboard Failure Logging (OBFL)
- Configuring SNMP
- Configuring NetFlow-lite
- Configuring Flexible NetFlow
- Configuring Ethernet OAM and CFM
- Configuring Y.1731 (AIS and RDI)
- Configuring Call Home
- Configuring Cisco IOS IP SLA Operations
- Configuring RMON
- Performing Diagnostics
- Configuring WCCP Version 2 Services
- Configuring MIB Support
- ROM Monitor
- Acronyms and Abbreviations
Catalyst 4500 Series Switch Software Configuration Guide, IOS XE 3.8.xE and IOS 15.2(4)Ex
Bias-Free Language
The documentation set for this product strives to use bias-free language. For the purposes of this documentation set, bias-free is defined as language that does not imply discrimination based on age, disability, gender, racial identity, ethnic identity, sexual orientation, socioeconomic status, and intersectionality. Exceptions may be present in the documentation due to language that is hardcoded in the user interfaces of the product software, language used based on RFP documentation, or language that is used by a referenced third-party product. Learn more about how Cisco is using Inclusive Language.
- Updated:
- January 7, 2015
Chapter: Configuring UDLD
- About UDLD
- Default UDLD Configuration
- Configuring UDLD on the Switch
- Fast UDLD Guidelines and Restrictions
- Enabling UDLD Globally
- Enabling UDLD on Individual Interfaces
- Disabling UDLD on Individual Interfaces
- Disabling UDLD on a Fiber-Optic Interface
- Configuring a UDLD Probe Message Interval Globally
- Enabling Fast UDLD Error Reporting
- Resetting Disabled LAN Interfaces
- Displaying UDLD Link Status
Configuring UDLD
This chapter describes how to configure UniDirectional Link Detection (UDLD) Ethernet on the switch, and includes the following major sections:
Note For complete syntax and usage information for the switch commands used in this chapter, see the
Cisco IOS Command Reference Guides for the Catalyst 4500 Series Switch.
If a command is not in the Cisco Catalyst 4500 Series Switch Command Reference , you can locate it in the Cisco IOS Master Command List, All Releases.
About UDLD
UDLD is a Layer 2 protocol that initializes devices connected through fiber-optic or twisted-pair Ethernet cables. This protocol monitors a physical connection (such as wrong cabling) to detect unidirectional links to avoid spanning-tree topology loops or silent drop traffic.
All connected devices must support UDLD for the protocol to successfully identify the unidirectional links. When UDLD detects a unidirectional link, it can administratively shut down the affected port and send you a warning message.
With UDLD, the time to detect a unidirectional link can vary from a few seconds to several minutes depending on how the timers are configured. Link status messages are exchanged every couple of seconds.
Starting with Cisco IOS Release 12.2(54)SG, the enhancement Fast UDLD was added, which supports timers in the few-hundred milliseconds range, which enables subsecond unidirectional link detection. With Fast UDLD, the time to detect a unidirectional link can vary from less than one second to a few seconds (the detection time also depends on how the timers are configured). Link status messages are exchanged every couple of hundred milliseconds.
This section includes these topics:
UDLD Topology
Each switch can send packets to a neighbor switch but cannot receive packets from the switch it is sending packets to. UDLD detects and disables these one-way connections. Figure 32-1 illustrates a unidirectional link condition.
Figure 32-1 Unidirectional Link Topology
Fast UDLD Topology
Figure 32-2 illustrates a typical Fast UDLD topology. Switch A and B are connected through a 2-port EtherChannel, and Fast UDLD is enabled on the individual ports. If one of the links becomes unidirectional, Fast UDLD detects this situation faster than regular UDLD and errdisables the link. Traffic is switched over to the second link by EtherChannel. Because this occurs very quickly, traffic loss is minimized.
Figure 32-2 Fast UDLD Topology
Note For Fast UDLD, Catalyst 4900M, Catalyst 4948E, Supervisor Engine 6-E, Supervisor 6L-E, Supervisor 7-E, and Supervisor Engine 7L-E support up to 32 ports.
Operation Modes
UDLD and Fast UDLD support the following operation modes:
- Normal—A UDLD-capable port (A) periodically sends a UDLD probe to a second port (B). If B is not UDLD capable, no unidirectional link detection occurs. If both devices are UDLD capable and bidirectional connectivity exists, probe messages travel in both directions at the rate of the configured message time interval. When the UDLD protocol receives the probe, it attempts to synchronize the devices by sending echo messages to the peer port and waiting for an answer during the detection window. If unidirectional traffic is detected when the port link is still up (B longer sends traffic to A), B enters errdisable mode, and A is marked undetermined but does not enter errdisable mode. It continues to operate under its current STP status because this mode is informational only; it is potentially less disruptive although it does not prevent STP loops.
Note Bidirectional link failures cannot be detected using normal mode.
- Aggressive—If a port (A) loses its neighbor connectivity, it actively attempts to reestablish the relationship by sending a probe to a second port (B). If port B does not respond, the link is considered unidirectional and port A enters an errdisable state to avoid silent drop traffic.
Note Both unidirectional and bidirectional link failures can be detected in aggressive mode.
UDLD aggressive mode can interoperate with UDLD normal mode. When a unidirectional condition is detected, only the aggressive mode link shuts down.
Default UDLD Configuration
Table 32-1 shows the UDLD default configuration.
Configuring UDLD on the Switch
The following sections describe how to configure UDLD:
- Fast UDLD Guidelines and Restrictions
- Enabling UDLD Globally
- Enabling UDLD on Individual Interfaces
- Disabling UDLD on Individual Interfaces
- Disabling UDLD on a Fiber-Optic Interface
- Configuring a UDLD Probe Message Interval Globally
- Resetting Disabled LAN Interfaces
Fast UDLD Guidelines and Restrictions
When using (or configuring) Fast UDLD, consider these guidelines and restrictions:
- Fast UDLD is disabled by default.
- Configure fast UDLD only on point-to-point links between network devices that support fast UDLD.
- You can configure fast UDLD in either normal or aggressive mode.
- Do not enter the link debounce command on fast UDLD ports.
- Configure fast UDLD on at least two links between each connected network device. This reduces the number of link disablements due to false positives.
- Fast UDLD does not report a unidirectional link if the same error occurs simultaneously on more than one link to the same neighbor device.
- Fast UDLD is supported on a limited number of ports.
Enabling UDLD Globally
To enable UDLD in aggressive or normal mode and to set the configurable message timer on all fiber-optic interfaces on the switch, perform this task:
Switch(config)# udld {aggressive | enable | message time message-timer-interval} | fast-hello error-reporting |
Specifies UDLD and Fast UDLD 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 “Operation Modes” section.
Note Prior to Cisco IOS Release 12.2(31)SGA, the timer range is 7 to 90 seconds. With |
|
Enabling UDLD on Individual Interfaces
To enable UDLD on individual interfaces, perform this task:
Disabling UDLD on Individual Interfaces
To disable UDLD on individual interfaces, perform this task:
Disabling UDLD on a Fiber-Optic Interface
To disable UDLD on individual fiber-optic interfaces, perform this task:
Configuring a UDLD Probe Message Interval Globally
To configure the time between UDLD probe messages on ports that are in advertisement mode and are currently determined to be bidirectional, perform this task:
Enabling Fast UDLD Error Reporting
By default, fast UDLD error-disables ports with unidirectional links. You can globally enable fast UDLD to report unidirectional links with a message displayed on the console instead of error-disabling ports with unidirectional links.
Note When fast UDLD error reporting is enabled, you must manually take the action appropriate for the state of the link.
To globally enable fast UDLD error reporting, perform this task:
Displaying UDLD Link Status
To verify link status reported by UDLD, enter the following command:
To verify status for a particular link as reported by UDLD, enter the following command:
To verify link status reported by Fast UDLD, enter the following command:
To verify status for a particular link as reported by Fast UDLD, enter the following command:
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