Understanding LBDT

LBDT periodically sends detection packets on an interface (see Detection Packet) to check whether the packets return to the local device (through the same interface or another interface), and determines whether a loop occurs on the interface, on the downstream network or device, or between two device interfaces.

If detection packets are received by the same interface, a loopback occurs on the interface or a loop occurs on the downstream network or device connected to the interface.
If detection packets are received by another interface on the same device, a loop occurs on the device or network connected to the interface.

Once a loop is detected, a Huawei switch sends a trap to the NMS and records a log. It then takes a preconfigured action on the interface (see Action Taken After a Loop Is Detected) to minimize the impact of the loop on the device and entire network.

When the device does not receive detection packets from the interface within the recovery time, the device considers that the loop is eliminated and restores the interface (see Automatic Recovery of an Interface).

Detection Packet

LBDT periodically sends detection packets on an interface to check whether the packets return to the local device to determine whether loops occur on the interface, on the downstream network or device, or between two device interfaces. The following conditions must be met:

Detection packets sent from an interface are sent back to the local device when a loop occurs on an interface, or network connected to the interface.
The system identifies detection packets sent from the local device, and detection packets that the interface sends.

Detection packets sent from a device carry the device's MAC address and outbound interface number. The device can identify the packets sent by itself as well as the source interface. The packets also carry the broadcast or multicast destination MAC address to ensure that the packets can be sent back to the local device when a loop occurs on the interface or network connected to the interface. Figure 1 shows the format of LBDT packets.

Figure 1 Format of LBDT packets

Table 1 describes the fields.

**Table 1** Description of each field
Item	Description
DMAC	The destination MAC address of a tagged packet is all Fs; the destination MAC address of an untagged packet is a BPDU MAC address, broadcast MAC address (all Fs), or multicast MAC address. The broadcast destination MAC address, multicast destination MAC address, or BPDU MAC address ensures that the detection packet can be sent back to the local device when a loop occurs on the interface or network connected to the interface.
SMAC	Source MAC address. The value is the system MAC address of the device, which identifies packets sent from the local device.
802.1Q Tag	Tag Protocol Identifier (TPID). The value of the TPID is 0x8100, representing the 802.1Q tagged frame.
LDT-Type	Detection packet type, including the protocol number and subprotocol number. The protocol number and subprotocol number are 0x9998 and 0x0001 respectively, indicating LBDT packets.
PortInfo	Information about the interface that sends detection packets, which is used by the device to determine whether packets are sent from the interface.
Flag	Untagged detection packet flag: 0x0003: indicates untagged packets. 0x0004: indicates tagged packets.

LBDT sends both tagged and untagged detection packets, so it can detect loops based on interfaces and VLANs.

Action Taken After a Loop Is Detected

When the system detects a loop on an interface, it can take a preconfigured action on the interface. Table 2 describes these preconfigured actions.

**Table 2** Actions taken after a loop is detected
Action	Description	Usage Scenario
Trap	The device only sends a trap to the NMS and records a log.	Select this action when only traps need to be reported without affecting traffic forwarding on the interface. This action cannot suppress broadcast storms.
Block	The device sends a trap to the NMS, blocks the interface, and allows only BPDUs to pass through.	Select this action when the interface needs to be disabled from forwarding data packets and needs to forward BPDUs such as Link Layer Discovery Protocol Data Units (LLDPDUs). This action can suppress broadcast storms.
Shutdown	The device sends a trap to the NMS and shuts down the interface.	Select this action to prevent broadcast storms when the interface does not participate in any calculation or forwarding. This action can suppress broadcast storms.
No learning	The device sends a trap to the NMS and disables the interface from learning new MAC addresses.	Select this action when the interface needs to process data packets and send them to the correct link. This action cannot suppress broadcast storms.
Quitvlan	The device sends a trap to the NMS and removes the interface from the VLAN where the loop occurs.	Select this action when loops in a VLAN need to be eliminated without affecting traffic forwarding in other VLANs. This action can suppress broadcast storms.

LBDT can only detect loops on a single node, but cannot eliminate loops on the entire network. After a loop is detected, you are advised to eliminate the loop immediately.

Automatic Recovery of an Interface

The automatic recovery mechanism of LBDT allows the looped interface to be restored immediately after a loop is eliminated. After the configured recovery time expires, the system attempts to restore the looped interface. If the device does not receive detection packets from the looped interface within the next recovery time, it considers that the loop is eliminated on the interface and restores the interface.

The interface that is disabled by LBDT cannot be restored after the recovery time. The interface that is manually shut down or enters the Error-Down state due to other protocols will be automatically restored after the recovery time. However, the interface that is disabled only by LBDT cannot be restored after the recovery time.
After the LBDT action of an interface is changed, the interface is restored. Then the changed LBDT action is taken when a loop is detected.
When VLAN-based LBDT is configured on an interface:
- If detection of this VLAN is canceled, the interface is restored automatically.
- If GVRP is not enabled on the interface and the interface is removed from the VLAN manually, the interface is restored automatically.
- If GVRP is enabled on the interface, the interface is manually removed from the VLAN or dynamically removed from the VLAN through GVRP, and the action to be taken is not shutdown, the interface can be restored automatically.
- If GVRP is enabled on the interface, the interface is manually removed from the VLAN or dynamically removed from the VLAN through GVRP, and the action to be taken is shutdown, in the alarm periodically reported by the device, the information about the VLAN where loops are detected is empty. For LBDT, You must run the shutdown and undo shutdown commands to manually restore the interface or run the restart command to enable the interface again. In addition to the preceding methods, the interface that is manually shut down or enters the Error-Down state due to other protocols will be automatically restored after the recovery time. However, the interface that is disabled only by LBDT cannot be restored after the recovery time.