Rapid STP & MSTP

Mental model

Classic 802.1D Spanning Tree (covered in Spanning Tree) prevents Layer-2 loops by blocking redundant links. It works, but it converges slowly:

Blocking     → Listening    → Learning    → Forwarding
   ↑ 20 s        ↑ 15 s          ↑ 15 s

Topology change → 30 seconds at minimum, often 50 seconds, before traffic flows. VoIP drops calls. Video stutters. Cloud apps reconnect. Unacceptable for modern networks.

RSTP (802.1w, 2001) keeps the loop-prevention math but reorganizes the state machine for fast convergence, typically 1-2 seconds, sub-second in trim topologies.

MSTP (802.1s, 2002) layers on top: it adds multi-instance support so you can have many VLANs share a small number of STP instances instead of running PVST+ (one STP per VLAN, which doesn’t scale beyond ~250 VLANs on most platforms).

Cisco gear runs PVST+ and Rapid-PVST+ (Cisco-flavored Per-VLAN STP) and MSTP (standards). In modern Cisco shops, Rapid-PVST+ is the default for small/mid deployments; MSTP is used in service-provider and very large enterprise.

RSTP: what changed

Port roles

Classic STP had four roles: Root, Designated, Blocking, Disabled. RSTP keeps Root and Designated and replaces the “blocking” idea with two more precise roles:

The key innovation is Alternate. RSTP has already pre-computed the next-best path; failover is just “promote Alternate → Root.” No timers, no recalculation.

Port states: only three

Classic STP had Blocking, Listening, Learning, Forwarding, Disabled (5 states). RSTP collapses them:

Blocking + Listening + Disabled all merged into Discarding.

Edge ports (PortFast)

RSTP formalizes the concept of an “edge port”: a port that connects to a host, not another switch. Edge ports skip the STP state machine entirely and go straight to Forwarding.

In Cisco IOS:

SW1(config-if)# spanning-tree portfast              ! single-port edge
SW1(config)# spanning-tree portfast default         ! all access ports are edge by default

If a switch BPDU arrives on an edge port, RSTP demotes it back to a normal port, protecting against loops if you accidentally cable two switches at an “edge” port. (Pair with BPDU Guard for hard error-disable. See BPDU Guard / Root Guard.)

Proposal / Agreement: the sync mechanism

When a new link comes up between two RSTP bridges, both could potentially carry traffic, but the bridge doesn’t know which side is closer to the root. Instead of waiting for timers, RSTP uses a proposal/agreement handshake:

1. Bridge A sends a Proposal BPDU on the new link: “I want this to be my designated port.”
2. Bridge B receives it, checks: am I better positioned? If A is closer to root, B agrees, but first syncs by temporarily Discarding on its other non-edge ports (to prevent a transient loop), then sends Agreement.
3. Bridge A puts the link into Forwarding.
4. B then propagates the same sync down its tree, fast, hop-by-hop.

Total convergence on a new link: hundreds of milliseconds to a couple of seconds, vs 30-50 seconds for legacy STP.

Configuration on Cisco

Cisco’s PVST+ runs one STP instance per VLAN. Rapid-PVST+ is the same but uses RSTP per VLAN:

SW1(config)# spanning-tree mode rapid-pvst

That’s it. No per-port changes needed for basic RSTP. All the role/state benefits kick in immediately.

MSTP: STP for many VLANs

PVST+ becomes painful at 100+ VLANs: one STP instance per VLAN means 100 separate calculations, 100 BPDU streams on every trunk. CPU and memory hit becomes real.

MSTP (802.1s) groups VLANs into a small number of instances. Instead of 100 STP processes, you might run 4, one per “VLAN class.” Each instance has its own root, port states, etc.

MST regions

All switches that share the same region name + revision + VLAN-to-instance mapping are in the same MST region. Inside the region, MSTP runs its full multi-instance logic. Between regions, MSTP looks like a single instance to legacy STP, so multi-vendor and legacy switches see one consistent STP world.

SW1(config)# spanning-tree mode mst
SW1(config)# spanning-tree mst configuration
SW1(config-mst)# name CAMPUS-A
SW1(config-mst)# revision 1
SW1(config-mst)# instance 1 vlan 10-99
SW1(config-mst)# instance 2 vlan 100-199
SW1(config-mst)# instance 3 vlan 200-299
SW1(config-mst)# end

Every switch in the region must have identical config (name + revision + mapping). One wrong character and the switch falls out of the region.

Why use MSTP

Two big wins:

1. Scale. 1000 VLANs → 4 instances instead of 1000 STP processes.
2. Traffic engineering. Make instance 1 use one set of trunks as primary, instance 2 use a different set. Both run active simultaneously. Real link-load balancing across redundant uplinks.

Cisco-only shops often stick with Rapid-PVST+, simpler. MSTP is mandatory in multi-vendor environments and in very dense enterprise with 200+ VLANs.

Quick comparison

RSTP-specific Cisco features

These are commonly tested as a layer on top of RSTP:

• PortFast: edge port, skip the state machine on link-up.
• BPDU Guard: if a PortFast port ever receives a BPDU, err-disable it. Hard loop prevention. (See BPDU Guard / Root Guard.)
• BPDU Filter: silently drop BPDUs on a port (rare, use with extreme caution).
• Root Guard: prevent an unexpected switch from claiming Root.
• Loop Guard: prevent unidirectional link failures from incorrectly transitioning a port to Forwarding.
• UDLD: unidirectional link detection at L1/L2, pairs with Loop Guard.

Verification

SW1# show spanning-tree summary
Switch is in rapid-pvst mode
...

SW1# show spanning-tree vlan 10
VLAN0010
  Spanning tree enabled protocol rstp
  Root ID    Priority    32778
             Address     001a.2b3c.4d5e
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Interface       Role Sts Cost      Prio.Nbr Type
  --------------- ---- --- --------- -------- --------------------
  Gi1/0/1         Desg FWD 4         128.1    P2p
  Gi1/0/2         Desg FWD 4         128.2    P2p Edge
  Gi1/0/24        Root FWD 4         128.24   P2p

SW1# show spanning-tree mst
SW1# show spanning-tree mst configuration
SW1# show spanning-tree mst 1 detail

The Type column tells you the link is P2p (point-to-point, full duplex switch-switch, RSTP can use fast handshake) vs Shared (half-duplex hub-style, must use the slow path).

Common mistakes

1. Leaving older platforms on their default pvst mode. You’re running classic 802.1D per VLAN: slow convergence. Change to rapid-pvst immediately.

2. PortFast on a switch-to-switch link. If anything other than a host plugs in, you’ve created a 1-second loop window. Pair with BPDU Guard always.

3. Mismatched MST configuration across the region. A typo in name, mismatched revision, or different VLAN-to-instance mapping → the switch falls out of the region, all its VLANs go to instance 0. Diagnose with show spanning-tree mst configuration digest.

4. Manual port priority instead of using bridge priority. New engineers tweak per-port priority to “fix” STP. Almost always wrong: the right tool is bridge priority on the right bridges (root and secondary root).

5. No backup root. Root bridge fails → tree rebuilds, but a secondary root that wasn’t pre-elected means slow re-election. Set spanning-tree vlan X root primary and root secondary on two bridges.

6. Mixing PVST+ and Rapid-PVST+ across switches. Mostly works (they fall back to common STP) but loses Rapid benefits. Standardize.

7. Forgetting half-duplex links revert to “Shared” type. RSTP can’t use proposal/agreement on a shared link → slow convergence on that segment. Always run full-duplex.

Lab to try tonight

1. Three switches in a triangle. All RPVST+: spanning-tree mode rapid-pvst.
2. Identify the Root: usually the one with the lowest MAC unless you’ve set priority.
3. Make SW1 root with spanning-tree vlan 1 priority 4096. Make SW2 secondary with priority 8192.
4. show spanning-tree vlan 1: observe port roles (Root, Designated, Alternate).
5. Shut down the link between SW2 and SW3. Time the convergence: ping should drop 1-2 packets, not 30+.
6. Unshut. Watch traffic shift back almost instantly.
7. Set up an end host on a switch port. spanning-tree portfast on that port: the interface comes up to forwarding instantly.
8. Bonus: convert all switches to MST. Region LAB, revision 1, map VLANs 10-19 to instance 1 and VLANs 20-29 to instance 2. Set SW1 as root for instance 1 and SW2 as root for instance 2 → independent active paths simultaneously.

Cheat strip