IEEE 802.1w defines Rapid Spanning Tree Protocol (RSTP), which preserves a loop-free Layer 2 topology while converging much faster than the original IEEE 802.1D STP. RSTP creates one spanning tree for the LAN; Cisco Rapid PVST+ applies the same rapid behavior separately to each VLAN. section 15.0 section 15.1
RSTP differs from traditional STP in several operational ways:
- Uses the same root bridge, root port, and designated port selection algorithm, but labels previously nondesignated ports as alternate or backup ports. section 15.2
- Combines STP blocking and listening into discarding; ports then use learning and forwarding as needed. section 15.2.2
- Uses synchronization between RSTP switches to move eligible ports directly from discarding to forwarding instead of waiting for timers. section 15.2.2
- Reacts after missing three BPDUs, normally about six seconds, rather than waiting for STP’s 20-second max-age period. section 15.2.4
Topology-change scenario
If a switch stops receiving BPDUs on its current root port without the interface going down, RSTP waits approximately six seconds, selects the next-best port, synchronizes with its neighbor, and forwards traffic. A directly detected interface failure can reduce downtime to less than one second. section 15.2.4
SW1(config)# spanning-tree mode rapid-pvst #1 SW1(config)# do show spanning-tree VLAN0001 Spanning tree enabled protocol rstp #2 . . . #1 Enables Rapid PVST+ #2 Rapid PVST+ is running.
On Cisco switches, configure Rapid PVST+ with spanning-tree mode rapid-pvst and verify it with show spanning-tree; the output identifies the active protocol as rstp. section 15.1
