Last Updated on August 1, 2021 by Admin 2
What are the three RSTP port states? (Choose three.)
Rapid Spanning Tree Protocol (RSTP) uses only three port states: discarding, learning, and forwarding. The learning and forwarding states are the same as the original STP standard, but the discarding state performs the functions originally performed in the disabled, blocking, and listening STP states.
With STP, you can safely assume that a listening port is either designated or root, and is on its way to the forwarding state. Unfortunately, once a port is in the forwarding state, there is no way to tell whether the port is root or designated. There is no difference in the operation of a port in blocking state and a port in listening state, since they both discard frames and do not learn MAC addresses. The real difference is in the role the spanning tree assigns to the port. RSTP decouples the role and the state of a port.
With RSTP, a role is assigned to a port. The root port and designated port roles are the same as with STP, while the blocking port role is split into the backup and alternative port roles. The Spanning Tree Algorithm (STA) determines the role of a port based on Bridge Protocol Data Units (BPDUs). The RSTP roles can be defined as follows:
- Root port: The port receiving the best BPDU on a bridge (lowest-cost path to the root bridge) is the root port.
- Designated port: The port that has the best path to the root bridge on a given segment is the designated port. The bridges connected to a given segment listen to each other’s BPDUs and agree on the bridge sending the best BPDU as the designated bridge for the segment. The corresponding port on that bridge is the designated port.
- Alternative port: An alternative port is a port blocked by receiving more useful BPDUs from another bridge. It becomes the root port if the active port fails.
- Backup port: A backup port is a port blocked by receiving more useful BPDUs originating from the same bridge. It becomes the designated port if the existing designated port fails.
Ports on the switch can also be classified as edge ports and non-edge ports. Access ports or edge ports are those that attach to devices such as workstations or printers. Non-edge ports are those that connect to other switches. If a non-edge port transitions to a forwarding state, a TC BPDU will be generated. On the other hand, when an edge ports transitions to the forwarding state, such as after a computer boots up or a device is connected to the port, no TC BPDU is generated.
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