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Configuration Guide - Ethernet Switching

S1720, S2700, S5700, and S6720 V200R012(C00 and C20)

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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Configuring Loop Prevention on a Port

Configuring Loop Prevention on a Port


On an RSTP network, a switch can only maintain the states of the root port and blocked ports if it is continuously receiving RST BPDUs from the upstream switch. If the ports cannot receive RST BPDUs from the upstream switch because of link congestion or unidirectional link failures, the switch re-selects a root port. Then, the previous root port becomes a designated port and the blocked ports change to the Forwarding state. As a result, loops may occur on the network. To prevent such a problem, configure loop protection.

When loop prevention is enabled, if the root port or alternate port does not receive BPDUs from the upstream device for a long time, the switch enabled with loop prevention sends a notification to the NMS. The root port enters the Discarding state and becomes the designated port, whereas the alternate port stays blocked and becomes the designated port. In this case, loops will not occur. When the link is no longer congested or unidirectional link failures are rectified, the port receives BPDUs for negotiation and restores its original role and status.


  1. Run system-view

    The system view is displayed.

  2. Run interface interface-type interface-number

    The view of the root port or alternate port is displayed.

  3. Run stp loop-protection

    Loop prevention is enabled on the root port or alternate port.

    By default, loop prevention is disabled on a port.


    An alternate port is a backup for a root port. If a device has an alternate port, configure loop prevention on both the root port and the alternate port.

    Root protection and loop prevention cannot be configured on the same port.

Updated: 2018-12-24

Document ID: EDOC1100038339

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