Application Scenarios for Loopback Detection
Using a Loopback Detection to Detect a Self-loop on an Interface
TX-RX (RX indicates the receiving end, and TX indicates the sending end) self-loops occur on an interface usually because optical fibers are connected incorrectly or the interface is damaged by high voltage. As shown in Figure 13-1, self-loops may occur on the network connected to a Switch interface. When a self-loop occurs, packets sent from the interface are sent back to this interface. This causes traffic forwarding errors or MAC address flapping on the interface.
Using a Loopback Detection to Detect a Loop on the Downstream Network
As shown in Figure 13-2, loops may occur on the network connected to a Switch interface. When a loop occurs, packets sent from the interface are sent back to this interface.
You can configure loopback detection on the interface of the Switch in the preceding scenarios. When a loopback is detected on the interface, the system sends an alarm. You can set the action to perform on an interface to error-down when a loopback is detected on the interface or set the time after which the interface in error-down state automatically recovers. Only users connected to the interface on which a loopback is detected and is in error-down state are affected, and other users connected to the Switch can still communicate.
- Loopback detection cannot prevent loops on the entire network. It only detects loops on a single node.
- A large number of packets are sent during loopback detection, occupying CPU resources; therefore, disable loopback detection if it is not required.
- Loopback detection cannot be used with ring network technologies including ERPS, Smart Link, STP, RSTP, MSTP, and VBST. Do not configure ring network technologies on an interface of the LBDT-enabled VLAN. If LBDT has been enabled globally and a ring network technology need to be configured on an interface, disable LBDT on that interface first.