Reference Guide
Version 8.3.19.0 Introduced on the S4820T.
Version 8.3.11.1 Introduced on the Z9000.
Version 8.3.10.1 Introduced the loopguard and rootguard options on the S4810.
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 shutdown-on-violation option.
Version 7.7.1.0 Introduced on the S-Series.
Version 7.5.1.0 Introduced on the C-Series.
Version 6.2.1.1 Introduced on the E-Series.
Usage
Information
If you enable portfast bpduguard on an interface and the interface receives a BPDU,
the software disables the interface and sends a message stating that fact. The port is in
ERR_DISABLE mode, yet appears in the show interface commands as enabled. If you do
not enable shutdown-on-violation, BPDUs are still sent to the RPM CPU.
STP loop guard and root guard are supported on a port or port-channel enabled in any
Spanning Tree mode: Spanning Tree Protocol (STP), Rapid Spanning Tree Protocol (RSTP),
Multiple Spanning Tree Protocol (MSTP), and Per-VLAN Spanning Tree Plus (PVST+).
Root guard is supported on any STP-enabled port or port-channel except when used as a
stacking port. When enabled on a port, root guard applies to all VLANs configured on the port.
STP 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
.
Do not enable Portfast BPDU guard and loop guard at the same time on a port. Enabling both
features may result 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.
To display the type of STP guard (Portfast BPDU, root, or loop guard) enabled on a port, enter
the show spanning-tree 0 command.
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