RSTP port roles and states
RSTP (IEEE 802.1w) redefines STP’s port roles, states, and BPDUs to fix the traditional protocol’s big weakness: convergence. 802.1D was designed for an era when networks tolerated 50-second redundancy delays; depending on the failure type, classic STP takes 30 to 50 seconds to converge after a change. RSTP greatly improves recalculation when the Layer 2 topology changes, including links coming up and indirect link failures.
Why the standard matters: proprietary mechanisms to speed up STP exist, but not every vendor or switch has them. RSTP is standards-based, so it works across vendor platforms, and it’s backward compatible with 802.1D, reverting to classic behavior per-port to interoperate with traditional switches.
Rapid PVST+ is to 802.1w what PVST+ is to 802.1D: a separate instance per VLAN.
How it gets fast
RSTP is proactive, which negates the need for 802.1D’s delay timers. Instead of waiting out timers, it negotiates port states with each peer switch through a proposal-and-agreement process:
One requirement: RSTP needs a full-duplex point-to-point connection between adjacent switches.
Most of the 802.1D terminology and parameters stay the same. You still configure a root bridge (and a backup root, per VLAN in Rapid PVST+); root ports and designated ports work as before.
Port roles
- Root port: on every nonroot bridge, the best path to the root. One per switch. Part of the active topology; forwards, sends, and receives BPDUs.
- Designated port: receives and forwards frames toward the root as needed. One per segment.
- Alternate port: offers an alternate path toward the root bridge. Discarding in the active topology; transitions to designated if the current designated port fails.
- Backup port: an additional port on the designated switch with a redundant link to a shared segment it’s already designated for. Discarding; moves to forwarding if the segment’s designated port fails.
- Disabled port: no role in spanning tree.
The change from classic STP: the nondesignated role is split into alternate and backup, which lets RSTP define a standby port before a failure happens. That’s the hot standby that makes sub-second failover possible.
You’ll probably never see a backup port in practice: it only occurs on shared segments, which require hubs, which are obsolete. The giveaway is a switch hearing its own BPDUs come back on a second port, meaning both ports are plugged into some unintelligent shared device, so one goes to discarding.
Port states
Three states, matching the three basic operations of a switch port. There is no listening state; listening and blocking are both replaced by discarding.
| STP role | STP state |
|---|---|
| Root port | Forwarding |
| Designated port | Forwarding |
| Nondesignated port | Blocking |
| Disabled | - |
| In transition | Listening, learning |
| RSTP role | RSTP state |
|---|---|
| Root port | Forwarding |
| Designated port | Forwarding |
| Alternate or backup port | Discarding |
| Disabled | Discarding |
| In transition | Learning |
- Discarding: seen in stable topologies and during changes. Prevents forwarding of data frames, breaking the continuity of a Layer 2 loop.
- Learning: seen in stable topologies and during changes. Accepts data frames to populate the MAC table, limiting unknown unicast flooding.
- Forwarding: stable active topologies only. Forwarding ports determine the topology; after a change, forwarding resumes only after proposal-and-agreement.
A port accepts and processes BPDUs in all states. That’s the difference from a classic blocked port: a discarding alternate port is still tracking costs and BPDUs, ready to fail over quickly.
Edge ports
RSTP’s version of PortFast. A port directly connected to an end station can’t create a bridging loop, so an edge port transitions straight to forwarding, skipping the intermediate states. Unlike PortFast, an edge port that receives a BPDU immediately loses its edge status and becomes a normal spanning tree port.
Enabling it
Catalyst switches default to PVST+, and moving to rapid is one command from global configuration:
Switch(config)# spanning-tree mode rapid-pvstChange it on all the Layer 2 switches. A mixed environment where one switch runs PVST+ falls back to the old, slow protocol on those links, since that’s the only version both sides agree on.