What is RSTP (Rapid Spanning Tree Protocol) and How to Configure RSTP

Advantages of RSTP over STP

The advantage of RSTP over STP is when we connect both switches in STP with two wires and one of the switches becomes the Root Bridge and the other becomes the Non-Root Bridge. One port on Non-Root Bridge becomes the Root port and the other port is on standby or shutdown. If the root port fails then the standby port becomes active / up.

RSTP over STP Configuration

The problems in STP are that how much time the standby port will take to become active? For this, after the failure of the root port, the standby port remains blocked for 20 seconds. This means the standby port automatically remains shut down after the root port failures. Then after, the BPDU starts listening for 15 seconds. BPDU is negotiating about both switches, and then it considers which port should be up.

The BPDU spends 15 seconds in the learning process (taking data) from switches. And then that port is placed into a Forwarding state. This process approximately takes 50 seconds, or around 1 minute for the standby port to be active.

BPDU Timeline

If we have multiple or above 100 switches so it goes late. It can disadvantage of having STP because the conversion will be slow and most of the time will spend on the standby port to the Forwarding state if the root port fails.

The IEEE addressed these problems and introduces the RSTP to overcome the disadvantages of STP.

The standard of STP is 802.1w defined by the committee standard IEEE, and it works on OSI Data-Link Layer. The RSTP is used to prevent loops and provide fast conversion.

What is RSTP

In RSTP, after the failure of the root port, the standby port remains to discard instead of blocking for 10 seconds which is better than the RSTP utilized time. Then after STP, the BPDU starts listening for 15 seconds, but the RSTP doesn’t believe in BPDU, the RSTP direct start learning for 5 seconds instead of listening BPDUs. And then that port is placed into a Forwarding state. This process approximately takes 15 seconds for the standby port to be active.

RSTP over STP BPDU Timeline

In other words, the STP considers the blocking state ports as “non-designated ports” while the RSTP considers discard ports as “alternate ports”. In STP, the ports (laptop or pc) connected in the Root Bridge are called “Designated Ports” while in RSTP these connected laptop or pc ports are called “Edge Ports”.

RSTP over STP Ports Name

By default, the STP is enabled on Cisco devices while we have to enable only the RSTP on those devices. The below command for enabling the RSTP will be applied in the third mode Config Mode.

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

Basic Switch Configuration STP Enabled

Switch 1

Switch>enable

Switch# configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

Switch (config) #hostname SW1

SW1 (config) #end

SW1# show spanning-tree

Show Spanning-Tree

SW1#

Switch 2

Switch>enable

Switch# configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

Switch (config) #hostname SW2

SW2 (config) #end

SW2# show spanning-tree

Show Spanning-Tree

SW2#

Enabling RSTP on Switches 3 and 4

Note: Enable the RSTP on Switch 3 and 4 to see the difference between STP and RSTP.

Switch 3

Switch>enable

Switch# configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

Switch (config) #hostname SW3

SW3 (config) # spanning-tree mode rapid-pvst

SW3 (config) #end

SW3# show spanning-tree

Spanning-Tree after Rapid-PVST

SW3#

Switch 4

Switch>enable

Switch# configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

Switch (config) #hostname SW4

SW4 (config) # spanning-tree mode rapid-pvst

SW4 (config) #end

SW4# show spanning-tree

Spanning-Tree after Rapid-PVST

SW4#

Now after doing the basic conversion, disconnect the cable from all switches and reconnect them and wait that how much time the switches will take to be up.

RSTP and STP Links up Time

Disconnect the fa 0/1 green port from the switches to see the result.

RSTP and STP Links up Time

After disconnecting the green ports the standby ports are active in the RSTP configured switch but in STP by default enabled switch is taking time to up the port.

Now the STP by default enabled switch standby port is active.