TCP vs UDP: Detail Explained
On the internet, two of the most important protocols are TCP and UDP. But what are they, and…
RSTP vs STP: What’s the Difference?
RSTP (Rapid Spanning Tree Protocol, IEEE 802.1w) is the modern replacement for the original STP (Spanning Tree Protocol, IEEE 802.1D). The fundamental difference is convergence speed — STP takes 30–50 seconds to recover from a topology change, while RSTP converges in 1–2 seconds. Both protocols prevent Layer 2 loops in networks with redundant switch connections, but RSTP does it much faster and more intelligently.
RSTP vs STP — Comparison Table
| Feature | STP (802.1D) | RSTP (802.1w) |
|---|---|---|
| IEEE Standard | 802.1D (1998) | 802.1w (2001), merged into 802.1D-2004 |
| Convergence Time | 30–50 seconds | 1–2 seconds |
| Port States | 5 (Blocking, Listening, Learning, Forwarding, Disabled) | 3 (Discarding, Learning, Forwarding) |
| Port Roles | Root, Designated, Non-Designated | Root, Designated, Alternate, Backup, Disabled |
| BPDU Generation | Only root bridge generates BPDUs | Every switch generates BPDUs every 2 seconds |
| Topology Change | Slow — max age timer must expire (20s) | Fast — immediate propagation via proposal/agreement |
| Edge Ports | PortFast (Cisco proprietary) | Edge Port (native in standard) |
| Compatibility | N/A | Backward compatible with STP |
| BPDU Version | Version 0 | Version 2 |
RSTP Port States vs STP Port States
STP has five port states. RSTP simplifies this to three by merging Blocking and Listening into a single Discarding state:
| STP State | RSTP Equivalent | Forwards Traffic? | Learns MACs? |
|---|---|---|---|
| Disabled | Discarding | No | No |
| Blocking | Discarding | No | No |
| Listening | Discarding | No | No |
| Learning | Learning | No | Yes |
| Forwarding | Forwarding | Yes | Yes |
RSTP Port Roles
RSTP introduces two new port roles not found in STP:
- Root Port: Best path to the root bridge — same as STP.
- Designated Port: Best port on each segment for forwarding — same as STP.
- Alternate Port (NEW): Backup path to the root bridge. Instantly becomes Root Port if the current Root Port fails — this is what enables RSTP’s fast convergence.
- Backup Port (NEW): Redundant connection to the same segment as the Designated Port. Rarely seen in practice.
- Disabled Port: Administratively shut down.
How RSTP Achieves Fast Convergence
RSTP uses a Proposal/Agreement mechanism to rapidly transition ports to Forwarding state without waiting for timers:
- A switch sends a Proposal BPDU to its neighbor indicating it wants to become the Designated Port.
- The downstream switch puts all its non-edge ports into Discarding state (sync).
- The downstream switch sends an Agreement BPDU back.
- The upstream switch immediately moves to Forwarding state.
- This process propagates hop by hop through the entire topology.
The entire process completes in under 2 seconds vs 30–50 seconds with STP timers.
RSTP Edge Ports vs STP PortFast
In STP, Cisco introduced PortFast as a proprietary feature to allow access ports to skip Listening and Learning states. RSTP standardizes this concept as Edge Ports — ports connected to end devices (PCs, phones, printers) that will never create a loop. Edge ports immediately enter Forwarding state. If an Edge port receives a BPDU, it automatically loses its edge port status (BPDU Guard can shut it down to prevent rogue switches).
When to Use RSTP vs STP
The answer is almost always RSTP. Use RSTP (Rapid PVST+ on Cisco) in all modern networks. STP’s 30–50 second convergence causes unacceptable outages. The only reason to keep STP is compatibility with very old legacy equipment that does not support RSTP — but RSTP is backward compatible with STP, so mixing the two in a network is safe.
Cisco Configuration: Enabling RSTP
! Enable Rapid PVST+ (Cisco's RSTP implementation per VLAN)
Switch(config)# spanning-tree mode rapid-pvst
! Verify
Switch# show spanning-tree | include mode
Switch# show spanning-tree summary
! Configure Edge Port (replaces PortFast)
Switch(config-if)# spanning-tree portfast
Switch(config-if)# spanning-tree bpduguard enableRSTP vs MSTP
While RSTP is faster than STP, it still runs one instance per VLAN in Cisco’s Rapid PVST+ implementation. MSTP (Multiple Spanning Tree Protocol, 802.1s) maps multiple VLANs to fewer STP instances, reducing CPU and memory overhead in networks with many VLANs. For networks with 20+ VLANs, MSTP is worth considering.
Frequently Asked Questions
Is RSTP backward compatible with STP?
Yes. If an RSTP switch receives an STP BPDU (version 0) from a neighbor, it automatically falls back to STP behavior on that port, ensuring compatibility with older switches.
Does RSTP completely replace STP?
Yes — the IEEE merged RSTP into the 802.1D-2004 standard, making RSTP the new “STP.” What is called STP today technically refers to RSTP in modern implementations.
What is the convergence time of RSTP?
RSTP convergence is typically 1–2 seconds using the Proposal/Agreement mechanism, compared to STP’s 30–50 seconds using timer-based convergence.
Related reading: STP Complete Guide | What is a Switch? | EtherChannel Guide
