Port roles and port states
RSTP keeps the root bridge election and adds explicit alternate and backup roles. An alternate port offers another path to the root. A backup port is redundant to a designated port on the same bridge and segment. Role describes a port’s job. State describes its data behavior as discarding, learning or forwarding.
How RSTP changes paths
On a point-to-point link, a proposal and agreement exchange lets bridges coordinate when a port can forward. A bridge first puts the other affected paths in a safe state. A configured host-facing edge port can go straight to forwarding. A link with a legacy STP neighbor can fall back to legacy behavior, so fast convergence depends on the link and the neighbor.
Try it in a lab
- Build a redundant triangle of RSTP-capable switches and record the root and alternate ports on a non-root switch.
- Disconnect its root link and observe the alternate port take over. Compare the role and state before and after the failure. In a packet trace, inspect the proposal and agreement flags on a point-to-point link as it comes up.