Portfast
As a CCNA / CCNP candidate you are expected to understand the purpose and function of Spanning-tree Portfast. The CCNA / CCNP’s exam will ask of you to determine the purpose of the Portfast within the spanning-tree protocol.
Before we go into the detail for the purpose and operation of the Portfast feature we need to understand the switched network environment behaviour before the introduction of portfast as a command.
As you may recall the Spanning-tree protocol is designed to prevent physical and/or logical loops in your layer 2 network. Spanning-tree achieves this loop free environment by first of all electing a device to act as the focal point of the network which all other switches measure themselves from. This focal point role is an elected role. The election of the focal point known as the “Root Bridge” is done at the very start of spanning-tree determining the loop free topology, the election of the Root Bridge is carried out by using a combination of the switches MAC address and a value known as the “Default Priority”.
From the “BPDU Guard” article you will recall that the stability of the Root Bridge is of paramount importance in the operation and continual uninterrupted service of spanning-tree and that a change in the position of the Root Bridge will cause service disruption on the network; another important factor to bear in mind is that is it not just the changing of the position of the Root Bridge that can cause network disruption but events such as access switchports transitioning on/off line can create an unstable network.
When a port first goes on-line spanning-tree will treat it like it treats any other port and will cycle through the normal spanning-tree port roles, i.e. blocking, listening. Learning, forwarding. The port will transition to the forwarding state after 30secs. In which no user traffic is allowed to pass.
At the point that the port went on-line the switch would have generated a new type of Bridge Protocol data unit called a “Topology Change Notification” BPDU, this TCN is relayed up out of the Root port towards the Root Bridge, the Root Bridge will register this TCN as a “Topology Change” and reset the TCN timer. These timers can be viewed by issuing the “show spanning-tree vlan # detail”. These false readings will cause no end of confusion in your fault finding and network disruption as you chase false positives.
The major benefit of Portfast on the access port, (since this command can only an should only be configured in access ports) is that it causes spanning-tree to bypass the lengthy forward delay timers on that port.
Port fast is enabled on an access port:
Swith(config-if)#spanning-tree portfast
Once PortFast is enabled it place the port directly into the forwarding state, bypassing the spanning-tree forward delay timers, but at the same time the PortFast feature will keep an eye open for any BPDU’s entering the access ports. If at any time the port received a BPDU it will default back to normal operation.
With the Portfast feature on the port the switch does not generate any more TCN’s when the port transitions on/off line, this means that any “Topology Changes” that you view from the output of the “show spanning-tree detail” you will see valid TCN’s along with the port that the TCN was received on.
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