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 per-port
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