Reference Guide

Version 8.4.2.1 Introduced the loopguard and rootguard options on the E-
Series TeraScale, C-Series, and S-Series.
Version 8.3.7.0 Introduced on the S4810.
Version 8.2.1.0 Introduced the hardware shutdown-on-violation option.
Version 7.6.1.0 Introduced on the S-Series.
Version 7.5.1.0 Introduced on the C-Series.
Version 7.4.1.0 Added the optional Bridge Port Data Unit (BPDU) guard.
Version 6.2.1.1 Introduced.
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 the Error Disable state if a BPDU appears, and a message is logged
so that the administrator can take corrective action.
NOTE: A port configured as an edge port, on a PVST switch, immediately transitions 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.
If you do not enable shutdown-on-violation, BPDUs are still sent to the route process
module (RPM) CPU.
You cannot enable root guard and loop guard 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#
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