Rapid Spanning Tree Protocol

802.1D did a decent job of preventing Layer 2 forwarding loops, but it used only one topology tree, which introduced scalability issues. Some larger environments with multiple VLANs need different STP topologies for traffic engineering purposes (for example, loadbalancing, traffic steering). Cisco created Per-VLAN Spanning Tree (PVST) and Per-VLAN Spanning Tree Plus (PVST+) to allow more flexibility.

PVST and PVST+ were proprietary spanning protocols. The concepts in these protocols were incorporated with other enhancements to provide faster convergence into the IEEE 802.1W specification, known as Rapid Spanning Tree Protocol (RSTP).

RSTP (802.1W) Port States

RSTP reduces the number of port states to three:

  • Discarding: The switch port is enabled, but the port is not forwarding any traffic to ensure that a loop is not created. This state combines the traditional STP states disabled, blocking, and listening.

  • Learning: The switch port modifies the MAC address table with any network traffic it receives. The switch still does not forward any other network traffic besides BPDUs.

  • Forwarding: The switch port forwards all network traffic and updates the MAC address table as expected. This is the final state for a switch port to forward network traffic.


A switch tries to establish an RSTP handshake with the device connected to the other end of the cable. If a handshake does not occur, the other device is assumed to be non-RSTP compatible, and the port defaults to regular 802.1D behavior. This means that host devices such as computers, printers, and so on still encounter a significant transmission delay (around 30 seconds) after the network link is established.

RSTP (802.1W) Port Roles

RSTP defines the following port roles:

  • Root port (RP): A network port that connects to the root switch or an upstream switch in the spanning-tree topology. There should be only one root port per VLAN on a switch.

  • Designated port (DP): A network port that receives and forwards frames to other switches. Designated ports provide connectivity to downstream devices and switches. There should be only one active designated port on a link.

  • Alternate port: A network port that provides alternate connectivity toward the root switch through a different switch.

  • Backup port: A network port that provides link redundancy toward the shared segment within the same collision domain, which is typically a network hub.

RSTP (802.1W) Port Types

RSTP defines three types of ports that are used for building the STP topology:

  • Edge port: A port at the edge of the network where hosts connect to the Layer 2 topology with one interface and cannot form a loop. These ports directly correlate to ports that have the STP portfast feature enabled.

  • Non-Edge port: A port that has received a BPDU.

  • Point-to-point port: Any port that connects to another RSTP switch with full duplex. Full-duplex links do not permit more than two devices on a network segment, so determining whether a link is full duplex is the fastest way to check the feasibility of being connected to a switch.


Multi-access Layer 2 devices such as hubs can only connect at half duplex. If a port can only connect via half duplex, it must operate under traditional 802.1D forwarding states.

Building the RSTP Topology

With RSTP, switches exchange handshakes with other RSTP switches to transition through the following STP states faster. When two switches first connect, they establish a bidirectional handshake across the shared link to identify the root bridge. This is straightforward for an environment with only two switches; however, large environments require greater care to avoid creating a forwarding loop. RSTP uses a synchronization process to add a switch to the RSTP topology without introducing a forwarding loop. The synchronization process starts when two switches (such as SW1 and SW2) are first connected. The process proceeds as follows:

  1. As the first two switches connect to each other, they verify that they are connected with a point-to-point link by checking the full-duplex status.

  2. They establish a handshake with each other to advertise a proposal (in configuration BPDUs) that their interface should be the DP for that segment.

  3. There can be only one DP per segment, so each switch identifies whether it is the superior or inferior switch, using the same logic as in 802.1D for the system identifier (that is, the lowest priority and then the lowest MAC address). Using the MAC addresses from Figure 2-1, SW1 (0062.ec9d.c500) is the superior switch to SW2 (0081.c4ff.8b00).

  4. The inferior switch (SW2) recognizes that it is inferior and marks its local port (Gi1/0/1) as the RP. At that same time, it moves all non-edge ports to a discarding state. At this point in time, the switch has stopped all local switching for non-edge ports.

  5. The inferior switch (SW2) sends an agreement (configuration BPDU) to the root bridge (SW1), which signifies to the root bridge that synchronization is occurring on that switch.

  6. The inferior switch (SW2) moves its RP (Gi1/0/1) to a forwarding state. The superior switch moves its DP (Gi1/0/2) to a forwarding state, too.

  7. The inferior switch (SW2) repeats the process for any downstream switches connected to it.

The RSTP convergence process can occur quickly, but if a downstream switch fails to acknowledge the proposal, the RSTP switch must default to 802.1D behaviors to prevent a forwarding loop.

Exam Preparation Tasks

As mentioned in the section “How to Use This Book” in the Introduction, you have a couple of choices for exam preparation: the exercises here, Chapter 30, “Final Preparation,” and the exam simulation questions in the Pearson Test Prep Software Online.