Rapid Spanning Tree Protocol

B.

The spanning-tree protocol (STP) is used in Ethernet networks to prevent loops and ensure there is only one active path between any two network devices. Spanning-tree enhancements have been introduced to speed up convergence time and provide faster failover mechanisms.

The enhancement that avoids the learning and listening states and immediately places ports in the forwarding state is called "PortFast" (answer B).

When a switch port is configured with PortFast, it is immediately placed in the forwarding state instead of going through the normal STP process of blocking, listening, and learning states. This is useful for switch ports connected to end-user devices, such as computers or printers, that do not participate in the STP and do not generate BPDUs (Bridge Protocol Data Units).

Without PortFast, when a user turns on their computer, the switch port initially enters the blocking state, then goes through the listening and learning states before finally transitioning to the forwarding state. This process takes about 30 seconds by default, which can be frustrating for end-users.

It is important to note that PortFast should only be used on access ports that connect to end-user devices. If PortFast is enabled on a switch port that is connected to another switch or a router, it can cause a loop in the network and disrupt communication.

The other spanning-tree enhancements listed as possible answers are:

• BPDUfilter (answer A): This feature is used to prevent BPDUs from being sent or received on a switch port. It is commonly used to protect against accidental loops caused by misconfigured network devices, but it should be used with caution as it can also prevent legitimate BPDUs from being received.
• Backbonefast (answer C): This feature is used to reduce the time it takes for a switch to recover from a link failure in the backbone of the network. It achieves this by sending special messages to neighboring switches to inform them of the failure and to ask them to bypass the normal STP process and immediately activate an alternate path.
• BPDUguard (answer D): This feature is used to protect against rogue switches or devices that may try to introduce a new root bridge or create a loop in the network. When enabled on a switch port, if a BPDU is received on that port, the switch automatically disables the port and sends an alert to the network administrator.