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Configuration Guide - Ethernet Switching

S7700 and S9700 V200R011C10

This document describes the configuration of Ethernet services, including configuring link aggregation, VLANs, Voice VLAN, VLAN mapping, QinQ, GVRP, MAC table, STP/RSTP/MSTP, SEP, and so on.
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Overview of LBDT and LDT

Overview of LBDT and LDT

LBDT and LDT periodically send detection packets through an interface to detect loops on the interface, on the downstream network or device, or between two device interfaces.

When a loop occurs on a network, broadcast, multicast, and unknown unicast packets are circulated on the network. This wastes network resources and can result in network breakdowns. Quickly detecting loops on a Layer 2 network is crucial for users to minimize the impact of loops on a network. LDT and LBDT help users check network connections and configurations, and control the looped interface.

LDT and LBDT periodically send detection packets on an interface 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. After a loop is detected, the device sends a trap to the NMS and records a log, and takes a preconfigured action on the looped interface (the interface is shut down by default) to minimize impact of the loop on the device and entire network.


LDT and LBDT can only detect loops on a single node, but cannot eliminate loops in the same manner as ring network technologies including ERPS, RRPP, SEP, Smart Link, STP, RSTP, MSTP, and VBST.

The switch support LDT and LBDT. Table 20-1 describes the differences between LDT and LBDT.

Table 20-1  Differences between LDT and LBDT




Detection packet

Sends only tagged detection packets, so loops are detected based on VLANs.

Sends tagged and untagged detection packets, so loops are detected based on interfaces and VLANs.

Action taken after a loop is detected

Provide Trap, Block, Shutdown, No Learning, and Quitvlan actions. (See Action Taken After a Loop Is Detected.)

Automatic recovery of the looped interface

Attempts to restore the interface when detection packets are not received within the recovery time.

Attempts to restore the interface after the recovery time. However, the interface shut down by LBDT cannot be restored.

Applicable scenario

Detect loopbacks on an interface, loops on the downstream network or device and loops between two device interfaces


  • Deployed at the aggregation layer.
  • Deployed based on VLANs.
  • Can detect loops on an Eth-Trunk.
  • Deployed at the access layer.
  • Deployed based on interfaces or VLANs.
  • Cannot detect loops on an Eth-Trunk.

Maximum number of VLANs that can be detected



Updated: 2019-10-18

Document ID: EDOC1000178310

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