US What RSTP is
RSTP (Rapid Spanning Tree Protocol) is the version of spanning tree that almost every managed Ethernet switch runs today. The IEEE published it as 802.1w in 2001, made it the only spanning tree in 802.1D-2004 and now maintains it within IEEE 802.1Q. Its job is the same as the original Spanning Tree Protocol: allow redundant links between switches while keeping exactly one active path, so frames cannot loop.
The difference is speed. Classic STP waits on fixed timers before a blocked port forwards, which adds up to 30 to 50 seconds after a failure. RSTP replaces most of that waiting with a direct handshake between neighboring switches and with backup ports worked out in advance. On a network of full-duplex point-to-point links, recovery typically takes a few hundred milliseconds to around a second.
RSTP still elects a root bridge, calculates path costs and blocks redundant links in the same way, so the shape of the tree is the same. It is also backward compatible: a port that hears an old 802.1D BPDU switches to classic STP behavior for that port only.
How RSTP converges faster
Four changes do most of the work.
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Proposal and agreement. When a link comes up between two switches, one proposes to forward, the other briefly blocks its own non-edge ports and agrees, and the link forwards at once. The handshake then moves outward switch by switch instead of waiting on timers.
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Edge ports. Ports that connect to end devices such as PLCs, HMIs and cameras are configured as edge ports and forward as soon as the link is up, without taking part in the handshake.
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Ready-made backup paths. Each switch keeps an alternate port that already knows a second route to the root. If the root port fails, the alternate takes over immediately.
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Faster detection and clean-up. Every switch sends BPDUs every 2 seconds, and a neighbour is declared lost after three missed hellos, around 6 seconds, or immediately when the link drops. Switches then flush the affected MAC table entries straight away.
STP vs RSTP
RSTP keeps the logic of STP but changes how a switch reaches a decision. The table compares them side by side.
| Feature | STP (802.1D-1998) | RSTP (802.1w) |
|---|---|---|
| Port states | Blocking, listening, learning, forwarding, disabled | Discarding, learning, forwarding |
| Port roles | Root, designated, blocked | Root, designated, alternate, backup, disabled |
| How a port starts forwarding | Waits out timers (2 × 15 s forward delay) | Proposal and agreement handshake with the neighbour |
| Detecting a lost neighbour | Up to 20 s (max age) | 3 missed hellos (around 6 s), or immediately on link loss |
| End-device ports | Vendor add-on (PortFast) | Edge ports defined in the standard |
| Topology change | Reported to the root, then flooded out | Any switch floods it, MAC tables flushed at once |
| Typical recovery | Around 30–50 s | Hundreds of ms to a few seconds |
In practice the answer to STP vs RSTP is almost always RSTP. The only reason to run classic STP is an old switch that supports nothing else, and RSTP talks to it automatically.
RSTP port roles and states
RSTP separates what a port is for, its role, from what it is doing, its state. That split is why it can react so quickly.
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Root port. The port with the best path to the root bridge. Every switch except the root has one.
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Designated port. The port that forwards traffic onto a segment, towards the edge of the tree.
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Alternate port. A discarding port with a second path to the root, ready to replace the root port.
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Backup port. A discarding port that backs up a designated port on the same segment, which only happens on shared media.
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Discarding, learning, forwarding. The three states. Discarding replaces STP’s blocking and listening, because neither forwards user traffic.
RSTP in industrial and SCADA networks
RSTP is the default redundancy protocol on most industrial managed switches, and it is often the common language between brands on a SCADA network, where HMIs, PLCs and historians share switches from several vendors. It suits meshed cores, dual-homed links and the uplinks that join plant cells.
Large rings are a different case. In a ring of many switches, RSTP recovery grows with the number of hops, and it can stretch to several seconds. That is why rings on production lines, roadside cabinets and rolling stock usually run a dedicated ring protocol, which our page on how ring topology works compares with spanning tree. The managed models among our M12 Ethernet switches for vehicles and our rack mount industrial switches for control rooms list RSTP and MSTP alongside ITU-T G.8032 ERPS, so a site can use both: ERPS on the rings and RSTP where they meet the rest of the network. Our white paper on types of rugged Ethernet switches covers the hardware side.
Four settings decide whether RSTP is actually rapid on site: edge ports on every device-facing port, point-to-point link type on full-duplex links between switches, a deliberately chosen root bridge, and no unmanaged switches in the middle of a redundant path.
Quick answers on RSTP
RSTP: common questions
What is RSTP used for?
Fast, loop-free redundancy on Ethernet. It lets switches have backup links and switch over to them within about a second if a cable or switch fails.
Is RSTP backward compatible with STP?
Yes, port by port. A port that receives a legacy 802.1D BPDU falls back to classic STP. That port then recovers at STP speed, so replace old switches where you can.
What is the difference between RSTP and MSTP?
One tree or several. RSTP builds a single tree for all VLANs. MSTP, IEEE 802.1s, runs several RSTP-style trees and maps groups of VLANs to each, so different VLANs can use different links.
How fast does RSTP converge?
Typically under a second on point-to-point links. It depends on the number of switches and on correct edge and link-type settings. A port that falls back to STP, or a long ring, takes longer.
What is an RSTP alternate port?
A pre-calculated backup path to the root. It discards traffic until the root port fails, then becomes the new root port immediately.
We supply industrial switches as part of a complete system, and as standard we configure the redundancy protocol, root priorities and edge ports to your network drawing before it ships.