According to Cisco's reference guide:
"When the Port Fast feature is enabled, the interface changes directly from a blocking state to a forwarding state without making the intermediate spanning-tree state changes."This basically means that the switch port automatically assumes a workstation will be attached, instead of another switch. Sounds great, huh? Everybody gets an immediate link light and an IP address.
Well, if a switch is attached to this port, it could create a topology loop and cause a data packet loop resulting in a network melt-down. But, another feature prevents the network melt-down worst case scenario, BPDU filter and guard.
BPDU is part of the Spanning-tree protocol. To learn more about BPDU, and Spanning-tree, visit Wikipedia - http://tinyurl.com/5gsycd, or this Cisco link - http://tinyurl.com/yqfvhw.
BPDU filter prevents the switch interface connected to end stations from sending or receiving BPDUs.
BPDU guard will place the interface that receives BPDUs into an error-disabled state. This prevents the possibility of creating a layer 2 loop.
So, let's get to the commands.
First, to activate the spanning-tree portfast feature, there is a couple of ways to do it:
- On a per port basis, the command is spanning-tree portfast.
- On a global switch basis, the command is spanning-tree portfast default.
Second, to activate BPDU filter in a global configuration mode, the command is spanning-tree portfast bpdufilter default.
Third, to activate BPDU guard in a global configuration mode, the command is spanning-tree portfast bpduguard default.
Next time, we will discuss a macro command that activates spanning-tree portfast and a couple of other features that should be used on each switch access port.