Caution

PortFast should be used only when connecting a single end station to a switch port. If PortFast enabled on a port connected to another networking device, such as a switch, you can create S When you enable PortFast on the Catalyst 3550, you will get the following message:

%Warning: PortFast should only be enabled on ports connected to a single host. Connecting hubs, concentrators, switches, bridges, etc. to this interface when PortFast is enabled can cause temporary bridging loops. Use with CAPUT ION

%Portfast has been configured on FastEthernet0/7 but will only have effect when the interface is in a nontrunking mode.

PortFast-enabled ports are still able to participate in STP and are still able to send and receive BPD PortFast port is indivertibly connected to another switch, an STP loop could result. Cisco implement that go hand in hand with PortFast to help prevent situations such as this: BPDU guard and BPDU 1

• BPDU guard— BPDU guard enforces the rule that PortFast-enabled ports should not receive ; a BPDU is received, that could indicate the port is connected to a switch and a possible STP lo result. A BPDU guard-enabled port puts the port in an error-disabled state if it receives a BPC BPDU guard i s disa bl ed on all interfa ces a nd sh ould be enablpd if PortFast is enabled. It can b globally, or on a single interface, with the following command:

3550_switch(config)#spanning-tree portfast bpduguard default

To enable or disab l e BDPU guard on an i nterface, use the following i nterface coccand:

3550_switch(config-if)#spanning-tree bpduguard [enable | disable]

You can verify BPDU guard with the show spanning-tree summary command.

3550_switch(config-if)#spanning-tree bpdufilter [enable | disable]

You can verify BPDU filtering by using the show spanning-tree detail command. At the end of th will see the number of BPDUs sent and received. The number received should always be t, and the should be small and not increment if BPDU filtering is enabled. Example 1-36 lists the output of the spanning-tree detail command after enabling PortFast with BPDU guard and BPDU filtering on in FastEfOsrnet t/ p.

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