22.1.1 Overview

UDLD(Uni-Directional Link Detection) is a function for detecting unidirectional link failure.

When a unidirectional link failure occurs, one switch is able to send data but cannot receive data, while the other switch is able to receive data but cannot send data.Furthermore, a malfunction occurs in an upper protocol, and various other failures occur throughout the network. Some of the known failures are loops in Spanning Tree Protocols and frame losses caused by link aggregation. These failures can be prevented by deactivating the applicable port when a unidirectional link failure is detected.

The OAM (Operations, Administration, and Maintenance) protocol, which functions as a part of the slow protocol in IEEE 802.3ah (Ethernet in the First Mile) and will be referred to hereafter as IEEE 802.3ah/OAM, describes the following method. OAM status information is regularly exchanged between the local switch and the partner switch by using control frames and checking frame-arrival capability at a remote device to monitor the bidirectional link status. The Switch uses the IEEE 802.3ah/OAM functionality to monitor the bidirectional link status. If the status cannot be checked in this case, UDLD functionality is used to detect unidirectional link failures. The UDLD functionality of this Switch determines that the Switch is in a loop configuration when it not only detects unidirectional link failures, but also when the Switch receives a control frame sent by itself and deactivates the port that received the frame.

The IEEE 802.3ah/OAM protocol also includes the concept of active and passive modes. The sending of a control frame starts at the active-mode switch and the passive-mode switch does not send any control frames until it has received a control frame. Because the factory default setting of the Switch enables IEEE 802.3ah/OAM functionality, all ports operate in passive mode.

Unidirectional link failures are detected by executing the efmoam active udld configuration command to configure the ports of both switches connected by an Ethernet cable. If a unidirectional link failure is detected on one of the ports configured with the efmoam active udld command, the port is deactivated and a link failure is detected on the port of the other switch. As a result, operations on the two ports of the connected switches are stopped.