CCIE Pursuit Blog

October 29, 2008

Question Of The Day: 29 October, 2008

Yesterday’s QoD:

Given the following configuration on a 3560 switch:

interface FastEthernet0/1
 switchport access vlan 101

You are tasked with ensuring that all communications are stopped on this port if the spanning-tree protocol is detected on this port.

Which of the following configuration choices best accomplishes this task:


Here are the results:

Answer %
spanning-tree bpduguard enable (interface) 85%
spanning-tree portfast bpduguard default (global)   7%
spanning-tree bpdufilter enable (interface) 6%
spanning-tree portfast bpdufilter default (global) 1%

The answer: spanning-tree bpduguard enable (interface)

Get comfortable with the “Configuring Optional Spanning-Tree Features” section of the 3560/3550 configuration guide as you are very likely to see a question or two relating these technologies on your practice labs.  In this case the technology that you want to use is bpduguard.  This feature will completely stop traffic on a port if there is a BPDU received on that port.  This command can be configured at the global or port level with a few differences in operation:

‘spanning-tree bpduguard enable’ under an interface will put the interface into error-disabled mode if a BPDU is received on that port.  The portfast feature does not need to be configured on the interface.

‘spanning-tree portfast bpduguard default’ at the global level will affect all ports on your switch that have the ‘portfast’ feature enabled.  Those ports will be shutdown if they receive a BPDU.

In our question, ‘portfast’ is not enabled on the interface so we need to use ‘spanning-tree bpduguard enable’ under the interface.

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