119supersedes the current legal root bridge, causing an undesired change of the network topology. Thetraffic that should go over high-speed links is switched to low-speed links, resulting in network congestion.To prevent this situation, MSTP provides the root guard function. If root guard is enabled on a port of aroot bridge, this port plays the role of designated port on all MSTIs. After this port receives aconfiguration BPDU with a higher priority from an MSTI, it performs the following actions:• Immediately sets that port to the listening state in the MSTI.• Does not forward the received configuration BPDU.This is equivalent to disconnecting the link connected with this port in the MSTI. If the port receives noBPDUs with a higher priority within twice the forwarding delay, it reverts to its original state.On a port, the loop guard function and the root guard function are mutually exclusive.Configure root guard on a designated port.To enable root guard:Step Command Remarks1. Enter system view. system-view N/A2. Enter Layer 2 Ethernet oraggregate interface view. interface interface-type interface-number N/A3. Enable the root guardfunction. stp root-protection By default, root guard isdisabled.Enabling loop guardBy continuing to receive BPDUs from the upstream device, a device can maintain the state of the root portand blocked ports. However, link congestion or unidirectional link failures might cause these ports to failto receive BPDUs from the upstream devices. In this situation, the device reselects the following port roles:• Those ports in forwarding state that failed to receive upstream BPDUs become designated ports.• The blocked ports transit to the forwarding state.As a result, loops occur in the switched network. The loop guard function can suppress the occurrence ofsuch loops.The initial state of a loop guard-enabled port is discarding in every MSTI. When the port receives BPDUs,it transits its state. Otherwise, it stays in the discarding state to prevent temporary loops.Do not enable loop guard on a port that connects user terminals. Otherwise, the port stays in thediscarding state in all MSTIs because it cannot receive BPDUs.On a port, the loop guard function is mutually exclusive with the root guard function or the edge portsetting.Configure loop guard on the root port and alternate ports of a device.To enable loop guard:Step Command Remarks1. Enter system view. system-view N/A2. Enter Layer 2 Ethernet oraggregate interface view. interface interface-type interface-number N/A
