120 Labs for Cisco CCNA 200-125 and CCENT Exams

Lab 125: Configuring Spanning-Tree PortFast

We know that Spanning-Tree Protocol (STP) help us to prevent to occur switching loop. However, spanning-tree sometime can also create a problem for your network. For example, suppose you have a server that is connected into your switch. You are sure that there are no chances to occur a switching loop even if STP is disabled. Participating in the STP process can prevent users from accessing the server resources until the STP convergence. Here comes a handy feature called spanning-tree PortFast. Using it means the port, on which this feature is enabled, would not participate in the STP process.

The ports enabled with the PortFast feature would not spend the usual 50 seconds to come up into the forwarding mode while STP is converging. Enabling the PortFast feature on all the switches will disable the STP process on all non-trunk ports. It will cause to take less time to change the state of the interfaces to up on all the non-trunking ports.

The following figure shows the commands used to enable PortFast. First, you need to select an interface for which you want to enable the PortFast feature and then execute the spanning-tree portfast command to enable this feature.

As shown in the preceding figure, the PortFast feature will only have effect when the interface is in a non-trunking mode. So enabling the PortFast feature on a trunk port is useless.

We will use the following topology to demonstrate how the PortFast feature works.

1.  Create the following topology but do not interconnect the devices now.

2.  Now, execute the following command on Switch to enable the PortFast feature on the Fa0/ 1 interface.

3.  Now connect PC0 to the fa0/ 1 interface and PC1 to the fa0/ 2 interface.

In the preceding figure, you will notice that the Fa0/ 1 interface will be activated within 5 seconds because it will not participate in the STP convergence process. But the Fa0/ 2 interface will take some time, usually 30-50 seconds, in order to come into the Forwarding state. Thas is what the PortFast feature does.

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