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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.
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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.
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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.
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