Overview of Loopback Detection

1 Overview
2 Detection Message
3 Loopback Detection Action

Overview

Loopback detection is a network technology used to identify and prevent switching loops within a network infrastructure. Switching loops occur when there is more than one path for data to travel between network devices, causing packets to continuously circulate and potentially overwhelm the network.

Loopback detection works by sending loopback detection messages out on interfaces and then monitoring for these messages to return. If the message returns to the originating switch, it indicates that a loop is present.

If the loopback detection message is received from the sending interface, it is considered that the interface is self-looped or there is a loop in the network connected to the interface.
If the loopback detection message is received by other interfaces on this device, it is considered that the network where the interface is located has a loop, or the device itself has a self-loop.

If a loop is detected, the switch takes action to mitigate it by disabling the affected interfaces and changing their state to err-disable. This helps maintain network stability and prevents performance degradation caused by excessive traffic looping.

NOTE:

The loopback detection message is sent only on the native VLAN. Upon receiving a loopback detection message, the device checks the source MAC address to determine if the message originated from itself, disregarding the VLAN tag.

Detection Message

The format of loopback detection message is depicted below.

DMAC

SMAC

802.1Q Tag

Protocol-Type

PortIndex

Field	Description
DMAC	The value of the destination MAC address is set to all Fs, regardless of whether the packet is tagged or untagged. This ensures that detection packets can be looped back to the device when a loop occurs on an interface or in the network.
SMAC	The source MAC address is set to the system MAC address of the local device to uniquely identify the packets sent by this device.
802.1Q Tag	It includes the Tag Protocol Identifier (TPID), with a value of 0x8100, indicating an 802.1Q tagged frame.
Protocol-Type	The protocol type field has a value of 0x9000, indicating a loopback detection message.
PortIndex	It represents the interface information of the sender of the detection message, allowing the device to compare interfaces and determine whether the message was sent from one of its own interfaces.

Loopback Detection Action

When the ingress interface receives a Loopback Detection (LBD) message sent from this device, then a loop is detected, the ingress interface will be set to the err-disable state, causing the physical link to go down. Additionally, the loop information will be logged.

You can clear the err-disable state of the interface through the following methods:

Use the command run clear loopback-detection interface to clear the err-disable state of the interface.
Delete the loopback detection configuration can clear the state of the interface.
Administratively reset the interface by disabling and then enabling it using the configuration commands.
Disable the loopback detection protocol to clear the state of the interface.

When loops occur within interfaces or networks, they can negatively impact routine business operations. Loopback detection, as a technology designed for detecting loops at a single node, possesses limited capabilities for resolving such loops. Consequently, it is recommended that users address loop-related problems promptly upon detection.