Rapid Spanning Tree Protocol (Cisco)

Technology: Switching
Area: RSTP
Vendor: Cisco
Software: 12.X , 15.X
Platform: Catalyst platforms

Rapid Spanning Tree Protocol also known as IEEE 802.1w is a newer version of STP that provides faster convergence than its predecessor. RSTP retain backward compatibility. Moreover, the terminology remains the same as the original 802.1D terminology, as well as most parameters, remained unchanged. The same spanning tree algorithm is used in both RSTP and STP, which allow us to determine port roles and topology. Configuration of RSTP should quite easy for those, who understand original STP technology.

You can check what version of STP you are running with the following command:

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

To change the STP mode use the global configuration command:

Switch(config)#spanning-tree mode ?
  mst         Multiple spanning tree mode
  pvst        Per-Vlan spanning tree mode
  rapid-pvst  Per-Vlan rapid spanning tree mode
Switch(config)#spanning-tree mode rapid-pvst

That’s it.

It’s worth to mention that RSTP increases the speed of the recalculation of the spanning tree in a case when the network topology of Layer 2 changes. As we mentioned in the beginning, RSTP offers much faster convergence in a correctly configured networks – in just a few hundred milliseconds. If the port is configured as an alternate port, it does not need to wait for network coverage, instead, it changes immediately to the forwarding state.

Please note that the Rapid PVST+ is a per-VLAN-based Cisco RSTP implementation. This means that an independent RSTP instance works for each VLAN.

The main differences between RSTP (802.1w) and the legacy STP(802.1d) are:

•  Port states disabled, blocking and listening in STP are superseded by discarding in RSTP,
• Two RSTP port roles: backup port and alternate port corresponds to the blocking state of STP,
• In STP only the root sends BPDU and relayed by others, while in RSTP all bridges generate BPDU every 2 sec, used as a „keepalive” mechanism.