Example for Configuring LBDT to Detect Loopbacks on an Interface

Overview

When a loop occurs on a network, broadcast, multicast, and unknown unicast packets are repeatedly transmitted on the network. This wastes network resources and may even cause a network breakdown. To minimize the impact of loops on a Layer 2 network, a detection technology that quickly notifies users of loops is required. When a loop occurs, users are requested to check network connections and configurations, and control the problematic interface.

The problematic interface continues to send LBDT packets. After the configured recovery time expires, the system attempts to restore the problematic interface. If the device receives no LBDT packets from the problematic interface within the next recovery time, it considers that the loop is eliminated on the interface and restores the interface.

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

Configuration Notes

• This example applies to all versions of all S series switches.

• In V200R008C00 and earlier versions, LBDT does not take effect in dynamic VLANs. In V200R008C00 and later versions, the LBDT-enabled switch can detect loops in dynamic VLANs, but the Quitvlan action is invalid for dynamic VLANs.

• LBDT needs to send a large number of LBDT packets to detect loops, occupying system resources. Therefore, disable LBDT if loops do not need to be detected.

• In versions earlier than V200R019C00, LBDT cannot be configured on an Eth-Trunk or its member interfaces. In V200R019C00 and later versions, LBDT can be configured on an Eth-Trunk but cannot be configured on its member interfaces.

• On the S5720-EI, S5720-HI, S5730-HI, S5731-H, S5731-S, S5731S-H, S5731S-S, S5732-H, S6720-EI, S6720S-EI, S6720-HI, S6730S-H, S6730-H, S6730-S, and S6730S-S, manual LBDT can be configured on a maximum of 64 Eth-Trunks; on other models, manual LBDT can be configured on a maximum of 32 Eth-Trunks.

• An interface can send LBDT packets with the specified VLAN tag only when the specified VLAN has been created.

• LBDT can detect loops in a maximum of eight VLANs.

• When the PVID of the interface in the loop is the detected VLAN ID or the interface joins the detected VLAN in untagged mode, VLAN tags of LBDT packets are removed. As a result, the packet priority changes and the system may fail to detect loops.

• When the Quitvlan action is used, the configuration file remains unchanged.

• The LBDT action and MAC address flapping action affect each other, and cannot be configured simultaneously.

• The Quitvlan action of LBDT conflicts with dynamic removal from VLANs (for example, GVRP and HVRP), and cannot be configured simultaneously.

• The blocked ports of LBDT cannot block GVRP packets. To ensure that GVRP runs normally and prevent GVRP loops, do not enable GVRP on the blocked port of LBDT.

• On a modular switch, LBDT and loop detection (LDT) cannot be configured simultaneously.

Networking Requirements

In Figure 1, aggregation switch SwitchA on an enterprise network connects to access switch SwitchB. To prevent loopbacks on a TX-RX interface (GE1/0/0) because optical fibers are connected incorrectly or the interface is damaged by high voltage, SwitchA is required to detect loopbacks on GE1/0/0. Furthermore, it is required that the interface be blocked to reduce the impact of the loopback on the network when a loopback is detected, and the interface be restored after the loopback is removed.

Figure 1 Networking for configuring LBDT to detect loopbacks on an interface

Configuration Roadmap

To detect loopbacks on downlink interface GE1/0/0 of SwitchA, configure LBDT on GE1/0/0 of SwitchA. The configuration roadmap is as follows:

1. Enable LBDT on GE1/0/0 of SwitchA to detect loopbacks.
2. Configure an action to be taken after a loopback is detected and set the recovery time. After a loopback is detected, the Switch blocks the interface to reduce the impact of the loopback on the network. After a loop is eliminated, the interface can be restored.