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S12700 V200R012C00 Configuration Guide - Ethernet Switching

This document describes the configuration of Ethernet services, including configuring MAC address table, link aggregation, VLANs, VLAN aggregation, MUX VLAN, VLAN termination, Voice VLAN, VLAN mapping, QinQ, GVRP, VCMP, STP/RSTP/MSTP, VBST, SEP, RRPP, ERPS, LBDT, and Layer 2 protocol transparent transmission.
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Enabling LDT

Enabling LDT


After global LDT is enabled in the system view, the system does not detect loops in any VLAN by default. To make LDT take effect, you must first enable global LDT and then enable LDT in a specified VLAN.

  • LDT needs to send a large number of detection packets to detect loops, occupying system resources. Therefore, run the undo loop-detection enable or undo loop-detection enable vlan command to disable LDT if loops do not need to be detected.

  • LDT cannot be used with ring network technologies of ERPS, RRPP, SEP, Smart Link, STP, RSTP, MSTP, and VBST. Do not configure ring network technologies on an interface of a LDT-enabled VLAN. In contrary, if LDT is enabled globally and ring network technologies need to be configured on an interface, run the loop-detection disable command to disable LDT on the interface.

  • The blocked ports of LDT cannot block GVRP packets. To ensure that GVRP runs normally and prevent GVRP loops, do not enable GVRP on the blocked port of LDT.
  • When loops occur in multiple VLANs on many interfaces, LDT performance is lowered due to limitations of security policies and CPU processing capability. The more VLANs and interfaces involved, the lower the performance is, especially the performance of the standby chassis in the cluster. Manually eliminating loops is recommended.


  1. Run system-view

    The system view is displayed.

  2. Run loop-detection enable

    Global LDT is enabled.

    By default, global LDT is disabled.

  3. Run interface interface-type interface-number

    The interface view is displayed.

  4. Select either of the following configurations to add the interface to the LDT-enabled VLANs.

    • Access interface

      1. Run port link-type access

        The link type of the interface is configured as access.

      2. Run port default vlan vlan-id

        The interface is added to the LDT-enabled VLANs.

    • Hybrid interface

      1. Run port link-type hybrid

        The link type of the interface is configured as hybrid.

      2. Run port hybrid tagged vlan { { vlan-id1 [ to vlan-id2 ] }&<1-10> | all }

        The hybrid interface is added to the LDT-enabled VLANs in tagged mode.

    • Trunk interface

      1. Run port link-type trunk

        The link type of the interface is configured as trunk.

      2. Run port trunk allow-pass vlan { { vlan-id1 [ to vlan-id2 ] }&<1-10> | all }

        The trunk interface is added to the LDT-enabled VLANs.

  5. Run quit

    Return to the system view.

  6. Run loop-detection enable vlan { { vlan-id1 [ to vlan-id2 ] } & <1-10> | all }

    LDT is enabled in VLANs.

    By default, LDT is disabled in all VLANs.

    LDT can detect loops in a maximum of 4094 VLANs

Updated: 2019-01-18

Document ID: EDOC1100038103

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