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Portfast and BPDU Guard

As useful as the STP process is, it can take some time to converge, which means that it can take a while for a blocked port to be unblocked and start forwarding traffic. This delay can cause issues in certain network environments, such as when using VoIP or other real-time applications on the devices connecting to the switches.

The reason for this behavior is the time STP spends for each stage the port transitions through until a stable topology is reached with no loops. In STP and PVST+, a port goes through the following stages:

  • Blocking: In this state, a port does not forward frames but listens to the network to prevent loops. A blocked port will not learn MAC addresses or send or receive frames.
  • Listening: In this state, a port is preparing to forward frames but is still listening to the network for any potential loops. During this stage, the port will still not forward frames or learn MAC addresses.
  • Learning: In this state, a port can learn MAC addresses and start building its forwarding table but still does not forward frames.
  • Forwarding: In this state, a port can forward frames between devices and actively participates in the network.
  • Disabled: In this state, a port has been manually disabled and does not participate in the network.

The PortFast feature can help improve network performance and reduce network convergence times when using STP. This is achieved by immediately transitioning a port from the blocking to the forwarding state when a device is connected. This can eliminate the delay during the STP convergence process and allow traffic to start flowing immediately.

However, it's important to note that PortFast should only be enabled on access ports connected to end devices such as computers or servers. It should not be enabled on ports connected to other switches or network devices, as this can cause loops in the network. To enable PortFast, you need to use the spanning-tree portfast command on the desired interface.

The problem with PortFast is that it can create a loop in the network if an unauthorized switch or other network device is connected to a port configured with PortFast. This is because PortFast does not participate in the STP learning process. 

A BPDU guard feature must be enabled on the same interface to protect against such behavior. BPDU guard is a security feature that helps to prevent unauthorized switches from disrupting the network topology and causing performance issues or downtime.

If an unauthorized device is connected to a port configured with PortFast, the BPDU guard will immediately disable the port, preventing any BPDU messages from being sent or received. This will prevent the loop from forming and protect the network from potential issues caused by the loop.

To enable BPDU guard, you need to configure the spanning-tree bpdufilter enable command in the interface configuration mode of the interface enabled for PortFast.