Rapid Spanning Tree Protocol (RSTP)

Rapid Spanning Tree Protocol (RSTP), defined in IEEE 802.1w, is an enhanced version of the original Spanning Tree Protocol (STP).

Its goal is simple:
reduce convergence time.

While STP can take up to 50 seconds to recover from a topology change, RSTP reduces this to a few hundred milliseconds.
Before enabling RSTP, let’s check which Spanning Tree mode the switch is currently running.

RSTP on Cisco Switches

On Cisco switches, RSTP is implemented as Rapid-PVST+.

This means:

• The protocol is based on IEEE 802.1w (RSTP)

• It runs one spanning tree instance per VLAN

• Each VLAN has its own independent Rapid Spanning Tree

So instead of having a single spanning tree for the whole switch, Cisco runs:

VLAN 1  → RSTP instance
VLAN 10 → RSTP instance
VLAN 20 → RSTP instance
...

Each VLAN converges independently and rapidly.

Enabling Rapid-PVST

Let’s enable RSTP on the switch.

SW2# conf t
Enter configuration commands, one per line.  End with CNTL/Z.
SW2(config)# spanning-tree mode rapid-pvst 
SW2(config)# end

Verifying the Mode

SW2# show spanning-tree summary

Switch is in rapid-pvst mode
Root bridge for: VLAN0001
Extended system ID                      is enabled
Portfast Default                        is disabled
Portfast Edge BPDU Guard Default        is disabled
Portfast Edge BPDU Filter Default       is disabled
Loopguard Default                       is disabled
PVST Simulation Default                 is enabled but inactive in rapid-pvst mode
Bridge Assurance                        is enabled
EtherChannel misconfig guard            is enabled
Configured Pathcost method used is short
UplinkFast                              is disabled
BackboneFast                            is disabled

Name                   Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
VLAN0001                     8         0        0          0          8
---------------------- -------- --------- -------- ---------- ----------
1 vlan                       8         0        0          0          8

The switch is now running in rapid-pvst mode, which confirms that RSTP is enabled.

On Cisco switches, RSTP is implemented as Rapid-PVST+, which means:

• It runs one spanning tree instance per VLAN

• Each VLAN has its own independent Rapid Spanning Tree

• Faster convergence applies independently to each VLAN

In our current topology:

• Only VLAN 1 exists

• The output shows VLAN0001

This confirms that Rapid-PVST+ is active for VLAN 1.

If additional VLANs were created (for example VLAN 10 or VLAN 20), each VLAN would automatically run its own Rapid Spanning Tree instance.

RSTP simplifies the port states used in STP by merging some of them:

STP vs. RSTP Port States

Listening is not a port state included in RSTP, it goes right from discarding to learning.

Figure 1 – Comparison of STP and RSTP Port States

While STP defines two primary port roles root and designated ports.

RSTP introduces two additional roles:

1. Alternate Port: Provides an alternative path to the root bridge.

   • Example: A redundant connection between switches remains ready for use if the primary link fails.
     

2. Backup Port: Acts as a backup for a shared medium like a hub.

   • Note: This role is less common today due to the rarity of hubs in modern networks.

Figure 2 – Comparison of STP and RSTP Port Roles