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| 3.4 |  | 802.1w Rapid Spanning-Tree Protocol |
| 3.4.2 |  | RSTP states |

802.1D defined five different port states:

- Disabled
- Listening
- Learning
- Blocking

Forwarding

This was a bit confusing because it mixed the state of a port (whether it blocks or forwards traffic) and the role it plays in the active topology (root port, designated port, and so on). For example, from an operational point of view, there is no difference between a port in Blocking state and a port in listening state. They both discard frames and do not learn MAC addresses. The real difference lies in the role the spanning tree assigns to the port. It can safely be assumed that a listening port is either designated or root and is on its way to the Forwarding state. Unfortunately, once in Forwarding state, there is no way to infer from the port state whether the port is root or designated, which contributes to demonstrating the failure of this state-based terminology. RSTP addresses this by decoupling the role and the state of a port.

There are only three port states left in RSTP corresponding to the three possible operational states. The 802.1D states disabled, blocking, and listening have been merged into a unique 802.1w Discarding state.

Rapid Transition to Forwarding state is the most important feature introduced by 802.1w. The legacy STA was passively waiting for the network to converge before turning a port into the Forwarding state. Achieving faster convergence was a matter of tuning the conservative default parameters (forward delay and max age timers), often putting the stability of the network at stake. The new rapid STP is able to actively confirm that a port can safely transition to forwarding without relying on any timer configuration. There is now a real feedback mechanism that takes place between RSTP-compliant bridges. In order to achieve fast convergence on a port, the protocol relies upon two new variables, which are edge ports and link type.

The edge port concept is already well known to spanning tree using Cisco protocol as it basically corresponds to the PortFast feature. All ports directly connected to end stations cannot create bridging loops in the network and can thus directly transition to forwarding, skipping the listening and learning stages. Neither edge ports nor PortFast enabled ports generate topology changes when the link toggles. Unlike PortFast, an edge port that receives a BPDU immediately loses its edge port status and becomes a normal spanning tree port. At this point, there is a user-configured value and an operational value for the edge port state. Cisco implementation maintains the PortFast keyword be used for edge port configuration, thus making the transition to RSTP simpler.

Link Type RSTP (point-to-point or shared) can only achieve rapid transition to forwarding on edge ports and on point-to-point links. The link type is automatically derived from the duplex mode of a port. A port operating in full-duplex will be assumed to be point-to-point, while a half-duplex port will be considered as a shared port by default. This automatic link type setting can be overridden by explicit configuration. In today's switched networks, most links are operating in full-duplex mode and are therefore treated as point-to-point links by RSTP. This makes them candidates for rapid transition to forwarding.