Rapid Spanning Tree Protocol (RSTP)

B

The spanning-tree protocol is used to prevent loops in a network by blocking redundant paths. When a switch is added to the network or a link goes down, the spanning-tree protocol recalculates the network topology, which can cause a delay in network traffic. The spanning-tree enhancements provide mechanisms to reduce this delay and improve network performance.

Among the four options listed, the spanning-tree enhancement that avoids the learning and listening states and immediately places ports in the forwarding state is PortFast.

PortFast is a Cisco proprietary feature that is used to bypass the traditional spanning-tree learning and listening states, which can cause unnecessary delays in network traffic. When a PortFast-enabled port is connected to an end device such as a PC or a server, the port immediately transitions from the blocking state to the forwarding state, which allows traffic to pass through the port without delay. This feature is particularly useful in access-layer switch ports that are connected to end devices.

However, PortFast should not be enabled on switch ports that are connected to other switches or routers because it can cause loops in the network. When a switch port that is not configured for PortFast is connected to a switch port that is configured for PortFast, the switch port with PortFast enabled assumes that it is connected to an end device and immediately transitions to the forwarding state. This can cause a temporary loop in the network until the spanning-tree protocol can recalculate the network topology and block the redundant path.

The other three options listed are also spanning-tree enhancements, but they do not immediately place ports in the forwarding state:

• BPDUfilter is used to filter out BPDU (Bridge Protocol Data Unit) frames, which are used by the spanning-tree protocol to exchange information between switches. This feature can be used to prevent loops in a network by blocking switch ports that receive BPDU frames from unauthorized switches or devices. If a switch port that is configured for BPDUfilter receives a BPDU frame, the port is immediately placed in the blocking state.

• Backbonefast is used to reduce the convergence time of the spanning-tree protocol when a switch loses its connection to the root bridge. This feature allows the switch to quickly identify an alternate path to the root bridge and transition the blocked ports to the forwarding state. However, this feature is only effective if there is an alternate path available.

• BPDUguard is used to prevent loops in a network by disabling switch ports that receive BPDU frames from unauthorized switches or devices. If a switch port that is configured for BPDUguard receives a BPDU frame, the port is immediately placed in the errdisable state, which disables the port until it is manually re-enabled by an administrator. This feature can be used to prevent rogue switches from being introduced into a network.