RSTP (802.1w): The same tree, a faster handshake
802.1D blocks a port and then just waits, stepping through timers because it has no way to know for certain the topology is actually safe yet. RSTP (Rapid Spanning Tree Protocol) replaces that waiting with an active conversation between neighboring switches: a proposal/agreement handshake, where a switch proposes "I'll be your designated port," the neighbor checks that its own ports are safe to synchronize, and agrees. That back-and-forth typically finishes in well under a second, instead of 30–50.
RSTP also adds two port roles beyond 802.1D's blocking/root/designated set, so a failover has less work to do:
- Alternate port: a backup path to the root, ready to take over instantly if the current root port fails.
- Backup port: a backup for a designated port on the same segment.
- Edge port: a port known to only ever face an end device (never another switch), so it can skip straight to forwarding with no negotiation needed at all.
Here's what those roles actually look like once they're assigned on a real three-switch topology, with one redundant link between SW2 and SW3:
| STP (802.1D) | RSTP (802.1w) | |
|---|---|---|
| How it converges | Passive, waits out fixed timers | Active, proposal/agreement handshake |
| Typical convergence | 30–50 seconds | Under a second |
| Port states | Blocking, Listening, Learning, Forwarding | Discarding, Learning, Forwarding |
| Extra roles | None | Alternate, Backup, Edge |
MST (802.1s): One tree isn't always the goal
Running a single spanning tree across every VLAN (Virtual Local Area Network) means every VLAN blocks the exact same link, which wastes the bandwidth of every other physical link for every VLAN that isn't using it. Cisco's PVST+ (Per-VLAN Spanning Tree Plus) solves that by running one full spanning tree instance per VLAN, so different VLANs can block different links and actually load-balance. But that means hundreds of VLANs can mean hundreds of spanning tree instances, all consuming CPU (Central Processing Unit) cycles and BPDUs (Bridge Protocol Data Units).
Multiple Spanning Tree (MST) is the middle ground: VLANs are grouped into a small number of instances, and each instance runs its own independent tree. You get the load-balancing benefit of per-VLAN trees, the CPU and BPDU overhead of only a handful of instances instead of hundreds.
In real deployments, RSTP (or a vendor's rapid variant of per-VLAN STP) is the default almost everywhere today. Plain 802.1D is mostly there so you recognize the port states and timers by name. MST only earns its complexity on networks with enough VLANs that per-VLAN instances would genuinely overload the switches.