DELL S50V User Manual

Rapid Spanning Tree Protocol | 911

The Loop Guard feature provides protection against Layer 2 forwarding loops (STP loops) caused by a 
hardware failure, such as a cable failure or an interface fault. When a cable or interface fails, a participating 
STP link may become unidirectional (STP requires links to be bidirectional) and an STP port does not 
receive BPDUs. When an STP blocking port does not receive BPDUs, it transitions to a forwarding state. 
This condition can create a loop in the network.

You enable loop guard on a per-port or per-port channel basis. 

To enable a loop guard on an RSTP-enabled port or port-channel interface, enter the 

 for more information on how to use the loop 

guard feature.

To disable RSTP loop guard on a port or port-channel interface, enter the 

 

command in an INTERFACE configuration mode. 

To verify the RSTP loop guard configuration on a port or port-channel interface, enter the 

spanning-tree rstp guard 

[

 

] 

command in global configuration mode.

FTOS Behavior: The following conditions apply to a port enabled with loop guard:
•

Loop guard is supported on any RSTP-enabled port or port-channel interface.

•

Loop guard is supported on a port or port-channel in any Spanning Tree mode:

 (STP)

 (RSTP)

 (MSTP)

•

Root guard and loop guard cannot be enabled at the same time on an RSTP port. For example, if you 
configure loop guard on a port on which root guard is already configured, the following error message is 
displayed: 

% Error: RootGuard is configured. Cannot configure LoopGuard.

•

Enabling Portfast BPDU guard and loop guard at the same time on a port results in a port that remains in a 
blocking state and prevents traffic from flowing through it. For example, when Portfast BPDU guard and 
loop guard are both configured:
- If a BPDU is received from a remote device, BPDU guard places the port in an err-disabled 

blocking state and no traffic is forwarded on the port.
- If no BPDU is received from a remote device, loop guard places the port in a loop-inconsistent 

blocking state and no traffic is forwarded on the port.

Enable loop guard on an RSTP-enabled port or port-channel 
interface.

INTERFACE

INTERFACE 
PORT-CHANNEL