Reference Guide
Per-VLAN Spanning Tree Plus (PVST+) | 1201
Usage
Information
The BPDU guard option prevents the port from participating in an active STP
topology in case a BPDU appears on a port unintentionally, or is misconfigured, or is
subject to a DOS attack. This option places the port into an error disable state if a
BPDU appears, and a message is logged so that the administrator can take corrective
action.
If
shutdown-on-violation is not enabled, BPDUs will still be sent to the RPM CPU.
Root guard and loop guard cannot be enabled at the same time on a 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.
When used in a PVST+ network, loop guard is performed per-port or per-port
channel at a VLAN level. If no BPDUs are received on a VLAN interface, the port or
port-channel transitions to a loop-inconsistent (blocking) state only for this VLAN.
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.
Example
FTOS(conf-if-gi-1/1)#spanning-tree pvst vlan 3 cost 18000
FTOS(conf-if-gi-1/1)#end
FTOS(conf-if-gi-1/1)#show config
!
interface GigabitEthernet 1/1
no ip address
switchport
spanning-tree pvst vlan 3 cost 18000
no shutdown
FTOS(conf-if-gi-1/1)#end
FTOS#
Related
Commands
Note: A port configured as an edge port, on a PVST switch, will immediately
transition to the forwarding state. Only ports connected to end-hosts should be
configured as an edge port. Consider an edge port similar to a port with a
spanning-tree portfast enabled.
show spanning-tree pvst View PVST+ configuration