Example for Configuring LBDT to Detect Loops on the Downstream Network

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, a new department of an enterprise connects to the aggregation switch Switch. This department belongs to VLAN 100. Loops occur due to incorrect connections or configurations. As a result, communication on the Switch and uplink network is affected.

It is required that the Switch should detect loops on the new network to prevent the impact of loops on the Switch and connected network.

Figure 1 Networking for configuring LBDT to detect loops on the downstream network

Configuration Roadmap

The configuration roadmap is as follows:

1. Enable LBDT on GE1/0/1 of the Switch to detect loops in a specified VLAN so that loops on the downstream network can be detected.
2. Set LBDT parameters so that the Switch can immediately shut down GE1/0/1 after a loop is detected. This prevents the impact of the loop on the Switch and connected network.

Configure interfaces on other switching devices as trunk or hybrid interfaces and configure these interfaces to allow packets from corresponding VLANs to pass through. This ensures Layer 2 connectivity on the new network and between the new network and the Switch.