What is the effect of configuring the following command on a switch?
Switch(config) # spanning-tree portfast bpdufilter default
A.
If BPDUs are received by a port configured for PortFast, then PortFast is disabled and the
BPDUs are processed normally.
B.
If BPDUs are received by a port configured for PortFast, they are ignored and none are
sent.
C.
If BPDUs are received by a port configured for PortFast, the port transitions to the
forwarding state.
D.
The command enables BPDU filtering on all ports regardless of whether they are
configured for BPDU filtering at the interface level.
Explanation:
Ordinarily, STP operates on all switch ports in an effort to eliminate bridging loops before
they can form. BPDUs are sent on all switch ports—even ports where PortFast has been
enabled. BPDUs also can be received and processed if any are sent by neighboring
switches. You always should allow STP to run on a switch to prevent loops. However, in
special cases when you need to prevent BPDUs from being sent or processed on one or
more switch ports, you can use BPDU filtering to effectively disable STP on those ports. By
default, BPDU filtering is disabled on all switch ports. You can configure BPDU filtering as a
global default, affecting all switch ports with the following global configuration command:
Switch(config)# spanning-tree portfast bpdufilter default
All ports that have PortFast enabled also have BPDU filtering automatically enabled. You
also can enable or disable BPDU filtering on specific switch ports by using the following
interface configuration command:
Switch(config-if)# spanning-tree bpdufilter {enable | disable}
Be careful when enabling BPDU filtering. Functionality is different when enabling on a perport basis or globally. When enabled globally, BPDU filtering is applied only on ports that are
in an operational PortFast state. Ports still send a few BPDUs at linkup before they
effectively filter outbound BPDUs. If a BPDU is received on an edge port, it immediately
loses its operational PortFast status and BPDU filtering is disabled.
Reference:
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.1E/native/command/reference/S1.html#wp1180453