- Index
- Preface
- Product Overview
- Command-Line Interfaces
- Configuring the Switch for the First Time
- Administering the Switch
- Configuring Interfaces
- Checking Port Status and Connectivity
- Configuring Supervisor Engine Redundancy Using RPR and SSO
- Configuring NSF with SSO Supervisor Engine Redundancy
- Configuring In Service Software Upgrade Process
- Configuring Control Plane Policing
- Environmental Monitoring and Power Management
- Configuring Power over Ethernet
- Configuring Switches with Cisco Network Assistant
- Configuring VLANs, VTP, and VMPS
- Configuring IP Unnumbered Interface
- Configuring Layer 2 Ethernet Interfaces
- Configuring SmartPort Macros
- Configuring STP and MST
- Configuring Optional STP Features
- Configuring EtherChannel
- Configuring IGMP Snooping and Filtering
- Configuring 802.1Q and Layer 2 Protocol Tunneling
- Configuring CDP
- Configuring UDLD
- Configuring Unidirectional Ethernet
- Configuring Layer 3 Interfaces
- Configuring Cisco Express Forwarding
- Configuring IP Multicast
- Configuring Policy-Based Routing
- Configuring VRF-lite
- Configuring QoS
- Configuring Voice Interfaces
- Configuring 802.1X Port-Based Authentication
- Configuring Port Security
- Configuring DHCP Snooping and IP Source Guard
- Configuring Dynamic ARP Inspection
- Configuring the AGM for the First Time
- Configuring Network Security with ACLs
- Configuring Private VLANs
- Configuring Port Unicast and Multicast Flood Blocking
- Configuring Port-Based Traffic Control
- Configuring SPAN and RSPAN
- Configuring System Message Logging
- Configuring SNMP
- Configuring NetFlow
- Configuring RMON
- Performing Diagnostics on the Catalyst 4500 Series Switch
- Configuring WCCPv2 Services
- Configuring MIB Support
- ROM Monitor
- Acronyms
Configuring UDLD
This chapter describes how to configure the UniDirectional Link Detection (UDLD) and Unidirectional Ethernet on the Catalyst 4500 series switch. It also provides guidelines, procedures, and configuration examples.
This chapter includes the following major sections:
•Configuring UDLD on the Switch
Note For complete syntax and usage information for the switch commands used in this chapter, refer to the Catalyst 4500 Series Switch Cisco IOS Command Reference and related publications at this location:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cr/index.htm.
Overview of UDLD
UDLD is a layer 2 protocol that enables devices connected through fiber-optic or twisted-pair Ethernet cables. It 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.
Modes of Operation
UDLD supports two operation modes: normal and aggressive.
normal A UDLD-capable port A periodically sends a UDLD probe to port B. If port B is not UDLD capable, no unidirectional link detection occurs. If both devices are UDLD capable and bi-directional connectivity exists, probe messages travel in both direction at a rate of 1 every few seconds (through the UDLD message time interval global configuration command). Upon receiving the probe, the UDLD protocol attempts to synchronize the devices by sending echo messages to the peer port and waiting for answer during the detection window. If the unidirectional traffic is detected when the port link is still up (port B no longer sends traffic to port A), port B enters errdisable mode. Port 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.
aggressive If port A loses its neighbor connectivity, it actively tries to re-establish the relationship by sending a probe to port B. If port B does not respond, it is considered unidirectional and port A will enter errdisable state to avoid silent drop traffic.
UDLD aggressive mode can interoperate with UDLD normal mode. When the unidirectional condition is detected, only the aggressive mode link shuts down.
By default,
•UDLD is locally disabled on copper LAN ports to avoid sending unnecessary control traffic (BPDU control packets) on this type of media; this protocol is most often used for access ports.
•UDLD is enabled on a fiber port if global UDLD is activated.
Figure 23-1 illustrates a unidirectional link condition. 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 23-1 Unidirectional Link
Default UDLD Configuration
Table 23-1 shows the UDLD default configuration.
Configuring UDLD on the Switch
The following sections describe how to configure UDLD:
•Enabling UDLD on Individual Interfaces
•Disabling UDLD on Non-Fiber-Optic Interfaces
•Configuring the UDLD Probe Message Interval
•Resetting Disabled LAN Interfaces
Enabling UDLD Globally
To enable UDLD in the aggressive or normal mode and to set the configurable message timer on all fiber-optic interfaces on the switch, follow these steps:
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Step 1 |
configure terminal |
Enters the global configuration mode. |
Step 2 |
udld {aggressive | enable | message time message-timer-interval} |
Specifies the UDLD mode of operation: •aggressive Enables UDLD in aggressive mode on all fiber-optic interfaces. •enable Enables UDLD in normal mode on all fiber-optic interfaces on the switch. UDLD is disabled by default. 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 section. •message time message-timer-interval-Configures the period of time between UDLD probe messages on ports that are in the advertisement phase and are determined to be bidirectional. The range is from 1 to 90 seconds. Note Prior to Cisco IOS Release 12.2(31)SGA, the timer range is 7 to 90 seconds. With |
Step 3 |
end |
Returns to privileged EXEC mode. |
Step 4 |
show udld |
Verifies the configuration. |
Enabling UDLD on Individual Interfaces
To enable UDLD on individual interfaces, perform this task:
Disabling UDLD on Non-Fiber-Optic Interfaces
To disable UDLD on individual non-fiber-optic interfaces, perform this task:
Configuring the UDLD Probe Message Interval
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:
Resetting Disabled LAN Interfaces
To reset all LAN ports that have been shut down by UDLD, perform this task:
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Switch(config)# udld reset |
Resets all LAN ports that have been shut down by UDLD. |