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10 Cisco Small Business 200, 300 and 500 Series Managed Switch Administration Guide (Internal Version) 146 Port Management: Unidirectional Link Detection This section describes how the Unidirectional Link Detection (UDLD) feature. It covers the following topics: •UDLD Overview •UDLD Operation •Usage Guidelines •Dependencies On Other Features •Default Settings and Configuration •Before You Start •Common UDLD Tasks •Configuring UDLD UDLD Overview UDLD is a Layer 2-protocol that enables devices connected through fiber-optic or twisted-pair Ethernet cables to detect unidirectional links. A unidirectional link occurs whenever traffic from a neighboring device is received by the local device, but traffic from the local device is not received by the neighbor. The purpose of UDLD is to detect ports on which the neighbor does not receive traffic from the local device (unidirectional link) and to shut down those ports.
Port Management: Unidirectional Link Detection UDLD Operation Cisco Small Business 200, 300 and 500 Series Managed Switch Administration Guide (Internal Version) 147 10 All connected devices must support UDLD for the protocol to successfully detect unidirectional links. If only the local device supports UDLD, it is not possible for the device to detect the status of the link. In this case, the status of the link is set to undetermined. The user can configure whether ports in the undetermined state are shut down or merely trigger notifications. UDLD Operation UDLD States and Modes Under the UDLD protocol, ports are assigned the following states: •Detection—System is attempting to determine whether the link is bidirectional or unidirectional. This is a temporary state. •Bidirectional—Traffic sent by a local device is known to be received by its neighbor, and traffic from the neighbor is received by the local device. •Shutdown—The link is unidirectional. Traffic sent by a local device is received by its neighbor, but traffic from the neighbor is not received by the local device. •Undetermined—The system cannot determine the state of the port, because of one of the following is occurring: -The neighbor does not support UDLD. or -The neighbor does not receive traffic from the local device. The UDLD action in this case depends on the UDLD mode of the device as explained below. UDLD supports the following modes of operation: •Normal -If the link is unidirectional, the port is shut down. -If the link is undetermined, the port is not shut down. Its status is changed to undetermined and a notification is sent. •Aggressive If the link is unidirectional or undetermined, the port is shut down.
Port Management: Unidirectional Link Detection UDLD Operation 148 Cisco Small Business 200, 300 and 500 Series Managed Switch Administration Guide (Internal Version) 10 UDLD is enabled on a port when one of the following occurs: •The port is a fiber port and UDLD is enabled globally. •The port is a copper port and you specifically enable UDLD on it. How UDLD Works When UDLD is enabled on a port, the following actions are performed: •UDLD initiates the detection state on the port. In this state, UDLD periodically sends messages on every active interface to all neighbors. These messages contain the device ID of all known neighbors. It sends these messages according to a user-defined message time. •UDLD receives UDLD messages from neighboring devices. It caches these messages until the expiration time (3 times message time) has passed. If a new message is received before the expiration time, the information in that message replaces the previous one. •When the expiration time expires, the device does the following with the information received: -If the neighbor message contains the local device ID—The link status of the port is set to bidirectional. -If the neighbor message does not contain the local device ID—T h e link status of the port is set to unidirectional, and the port is shut down. •If UDLD messages are not received from a neighboring device during the expiration time frame, the link status of the port is sent to undetermined and the following occurs: -Device is in normal UDLD mode: A notification is issued. -Device is in aggressive UDLD mode. The port is shut down. While the interface is in the bidirectional or the undetermined state, the device periodically sends a message each message time seconds. The above steps are performed over and over. A port that was shut down can be reactivated manually in the Port Management > Error Recovery Settings page. For more information, see Reactivating a Shutdown Port.
Port Management: Unidirectional Link Detection UDLD Operation Cisco Small Business 200, 300 and 500 Series Managed Switch Administration Guide (Internal Version) 149 10 If an interface is down and UDLD is enabled, the device removes all neighbor information and sends at least one ULDL message to the neighbors informing them that the port is down. When the port is brought up, the UDLD state is changed to detection. UDLD Not Supported or is Disabled on a Neighbor If UDLD is not supported or disabled on a neighbor, then no UDLD messages are received from that neighbor. In this case, the device cannot determine whether the link is unidirectional or bidirectional. The status of the interface is then set to undetermined. The actions taken by the device depend on whether the UDLD mode is normal or aggressive. Inconsistent UDLD Mode in Local and Neighboring Device It is possible for the local device and its neighbor to be set to a different UDLD mode (normal, aggressive). The UDLD mode is not contained in the UDLD messages, so that the local device does not know the UDLD mode of the neighbor and vice versa. If the UDLD modes are different on the local and neighbor device, the devices act as follows: •When the UDLD state of the link is bidirectional or unidirectional, both devices shut down their ports. •When the UDLD state of the port is undetermined, the side with the normal UDLD mode merely issues a notification, while the side with the aggressive UDLD mode shuts down the port. If both devices are in normal mode, the port is not shut down when its state is undetermined. Reactivating a Shutdown Port You can reactivate a port that was shut down by UDLD in one of the following ways: •Automatically—You can configure the system to automatically reactivate ports shut down by UDLD in the Port Management > Error Recovery Settings page. In this case, when a port is shut down by UDLD, it is automatically reactivated when the automatic recovery interval expires.
Port Management: Unidirectional Link Detection Usage Guidelines 150 Cisco Small Business 200, 300 and 500 Series Managed Switch Administration Guide (Internal Version) 10 UDLD again begins running on the port. If the link is still unidirectional, UDLD shuts it down again after the UDLD expiration time expires, for instance. •Manually—You can reactivate a port in the Port Management > Error Recovery Settings page Usage Guidelines Cisco does not recommend enabling UDLD on ports that are connected to devices on which UDLD is not supported or disabled. Sending UDLD packets on a port connected to a device that does not support UDLD simply causes more traffic on the port without providing benefits. In addition, take the following into consideration when configuring UDLD: •Set the message time according to how urgent it is to shut down ports with a unidirectional link. The lower the message time, the more UDLD packets are sent and analyzed, but the sooner the port is shut down if the link is unidirectional. •If you want UDLD to be enabled on a copper port, you must enable it per port. When you globally enable UDLD, it is only enabled on fiber ports. •Set the UDLD mode to normal when you do not want to shut down ports unless it is known for sure that the link is unidirectional. •Set the UDLD mode to aggressive when you want to shut down any port whenever there is even a chance that the link is undetermined. Dependencies On Other Features •UDLD and Layer 1. When UDLD is enabled on a port, UDLD actively runs on that port while the port is up. When the port is down, UDLD goes into UDLD shutdown state. In this state, UDLD removes all learned neighbors. When the port is changed from down to up, UDLD resumes actively running.
Port Management: Unidirectional Link Detection Default Settings and Configuration Cisco Small Business 200, 300 and 500 Series Managed Switch Administration Guide (Internal Version) 151 10 •UDLD and Layer 2 Protocols UDLD runs on a port independently from other Layer 2 protocols running on the same port, such as STP or LACP. For example, UDLD assigns the port a status regardless of the STP status of the port or regardless of whether the port belongs to a LAG or not. Default Settings and Configuration The following defaults exist for this feature : •UDLD is disabled by default on all ports of the device. •Default message time is 15 seconds. •Default expiration time is 45 seconds (3 times the message time). •Default port UDLD state: -Fiber interfaces are in the global UDLD state. -Non-fiber interfaces are in the disable state. Before You Start No preliminary tasks are required. Common UDLD Tasks This section describes some common tasks to setup UDLD. Work flow1: To globally enable UDLD on fiber por ts, perform the following steps: STEP 1Open the Port Management > UDLD Global Settings page. a. Enter the Message Time. b. Select either Disabled, Normal or Aggressive as the global UDLD status.
Port Management: Unidirectional Link Detection Configuring UDLD 152 Cisco Small Business 200, 300 and 500 Series Managed Switch Administration Guide (Internal Version) 10 STEP 2Click Apply Workflow2: To change the UDLD configuration of a fiber por t or to enable UDLD on a copper por t, perform the following steps: STEP 1Open the Port Management > UDLD Global Settings page. a. Select a port. b. Select either Default, Disabled, Normal or Aggressive as the port’s UDLD status. If you select Default, the port receives the global setting. STEP 2Click Apply. Workflow3: To bring a por t up after it was shut down by UDLD and automatic reactivation was not configured: STEP 1Open the Port Management > Error Recovery Settings page. a. Select a port. b. Click Reactivate. Configuring UDLD The UDLD feature can be configured for all fiber ports at one time (in the UDLD Global Settings page) or per port (in the UDLD Interface Settings page). UDLD Global Settings The Fiber Port UDLD Default State is only applicable to fiber ports. The Message Time field is applicable to both copper and fiber ports. To configure UDLD globally: STEP 1Click Port Management > UDLD > UDLD Global Settings. STEP 2Enter the following fields: •Message Time—Enter the interval between two sent UDLD messages. This field is relevant for both fiber and copper ports.
Port Management: Unidirectional Link Detection Configuring UDLD Cisco Small Business 200, 300 and 500 Series Managed Switch Administration Guide (Internal Version) 153 10 •Fiber Port UDLD Default State—This field is only relevant for fiber ports. The UDLD state of copper ports must be set individually in the UDLD Interface Settings page. The possible states are: -Disabled—UDLD is disabled on all ports of the device. -Normal—Device shuts down an interface if the link is unidirectional. If the link is undetermined, a notification is issued. -Aggressive—Device shuts down an interface if the link is unidirectional or undetermined. STEP 3Click Apply to save the settings to the Running Configuration file. UDLD Interface Settings Use the UDLD Interface Settings page to change the UDLD state for a specific port. Here the state can be set for copper or fiber ports. To copy a particular set of values to more than one port, set that value for one port and use the Copy button to copy it to the other ports. To configure UDLD for an interface: STEP 1Click Port Management > UDLD > UDLD Interface Settings. Information is displayed for all ports on which UDLD is enabled, or, if you have filtered only a certain group of ports, information is displayed for that group of ports. •Port—The port identifier. •UDLD State—T h e p o s s i b l e s t a t e s a r e : -Disabled—UDLD is disabled on all fiber ports of the device. -Normal—Device shuts down an interface if it detects that the link is unidirectional. It issues a notification if the link is undetermined. -Aggressive—Device shuts down a port if the link is unidirectional or undetermined. •Bidirectional State—Select the value of this field for the selected port. The possible states are: -Detection—The latest UDLD state of the port is in the process of being determined. Expiration time has not yet expired since the last
Port Management: Unidirectional Link Detection Configuring UDLD 154 Cisco Small Business 200, 300 and 500 Series Managed Switch Administration Guide (Internal Version) 10 determination (if there was one), or since UDLD began running on the port, so that the state is not yet determined. -Bidirectional—Traffic sent by the local device is received by its neighbor, and traffic from the neighbor is received by the local device. -Undetermined—The state of the link between the port and its connected port cannot be determined either because no UDLD message was received or the UDLD message did not contain the local device ID in it. -Disabled—UDLD has been disabled on this port. -Shutdown—The port has been shut down because its link with the connected device is unidirectional or undetermined in aggressive mode. •Number of Neighbors—Number of connected devices detected. STEP 2To modify the UDLD state for a specific port, select it and click Edit. STEP 3Modify the value of the UDLD state. If you select Default, the port receives the value of the Fiber Por t UDLD Default State in the Global UDLD Settings page. STEP 4Click Apply to save the settings to the Running Configuration file. UDLD Neighbors To view all devices connected to the local device: STEP 1Click Port Management > UDLD > UDLD Neighbors. The following fields are displayed for all UDLD-enabled ports. •Interface Name—Name of the local UDLD-enabled port. •Neighbor Information: -Device ID—ID of the remote device. -Device MAC—MAC address of the remote device. -Device Name—Name of the remote device. -Por t ID—Name of the remote port.
Port Management: Unidirectional Link Detection Configuring UDLD Cisco Small Business 200, 300 and 500 Series Managed Switch Administration Guide (Internal Version) 155 10 •State—State of the link between the local and neighboring device on the local port. The following values are possible: -Detection—The latest UDLD state of the port is in the process of being determined. Expiration time has not yet expired since the last determination (if there was one), or since UDLD began running on the port, so that the state is not yet determined. -Bidirectional—Traffic sent by the local device is received by its neighbor, and traffic from the neighbor is received by the local device. -Undetermined—The state of the link between the port and its connected port cannot be determined either because no UDLD message was received or the UDLD message did not contain the local device ID in it. -Disabled—UDLD has been disabled on this port. -Shutdown—The port has been shut down because its link with the connected device is unidirectional or undetermined in aggressive mode. •Neighbor Expiration Time (Sec)—Displays the time that must pass before determining the port UDLD status. This is three times the Message Time. •Neighbor Message Time (Sec)—Displays the time between UDLD messages.