Which two statements are true about STP and RSTP? (Choose two.)
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A. B. C. D.BD
STP (Spanning Tree Protocol) and RSTP (Rapid Spanning Tree Protocol) are network protocols used to prevent loops in a network topology. Both protocols operate by selecting a root bridge, and then placing each switch port into either forwarding or blocking state, based on a set of rules that ensure a loop-free topology. Here are the two true statements about STP and RSTP:
A. STP uses a proposal-and-agreement handshake on point-to-point links instead of timers: In STP, the process of electing a root bridge is based on the lowest bridge ID, and ties are broken based on the lowest port ID. Once the root bridge is elected, the other switches calculate their distances to the root bridge and select the root ports (the port that provides the shortest path to the root bridge) and designated ports (the port that connects to the lowest cost path to the root bridge). In point-to-point links, instead of waiting for timers, STP uses a proposal-and-agreement handshake to quickly select the root port. The switch with the lowest bridge ID sends a proposal message to its neighbor, asking to become the root port. If the neighbor agrees, it sends an agreement message, and the port immediately transitions to forwarding state. This eliminates the need to wait for the expiration of timers, and speeds up the convergence of the network.
B. In RSTP, root ports and edge ports transition to the forwarding state immediately, without exchanging messages with other switches: RSTP is an evolution of STP that was designed to reduce the convergence time and improve the performance of the network. One of the main differences between RSTP and STP is that RSTP introduces two new port states: the discarding state and the alternate state. The discarding state replaces the blocking state of STP, and allows the port to still listen and learn from incoming packets, without forwarding them. The alternate state is a backup state that provides an alternate path to the root bridge, in case the primary path fails. When a switch detects a topology change, it sends a Topology Change Notification (TCN) message to the root bridge, which then floods the network with TCN messages. In RSTP, root ports and edge ports (ports that connect to end hosts) transition to the forwarding state immediately, without waiting for other switches to send TCN messages. This speeds up the convergence of the network, and reduces the downtime of the network.