What is Rapid Spanning Tree Protocol

In This section we will cover the Following Questions

• What is RSTP?
• what are switching features like?
• Portfast
• Uplinkfast
• Backbonefast
• What are the Port Status in RSTP?
• What is Convergence time of STP Vs RSTP ?
• How to configure RSTP?

So Let's start with 

What is RSTP ?

• Rapid spanning tree protocol is a IEEE  ( 802.1w ) standard protocol 
• IT is designed to speed up STP convergence on access port.
• RSTP is by default enabled with portfast feature.
• RSTP is by default enabled with uplinkfast feature
• Path calculation is remain same as STP
• RSTP has the combined feature of portfast, uplinkfast and backbonefast which are cisco proprietary 

Note : STP and RSTP both are the same, but if it is needed to speedup STP, RSTP has to be enabled 

Portfast :

• Portfast is cisco proprietary  protocol , 
• It's used to minimize the impact of STP, TCN, BPDU traffic when a simple host is being rebooted or connected to a switch. 
•  It's a Layer 2 function

 As we all know that Port status in STP are

•  Disable------
• Blocking
• Listening (15 sec)
• Learning(15 sec)
• Forwarding

In normal condition on the startup of switch it goes through the process of listening , learning and then forwarding  which  take minimum of 30 sec to come into a forwarding state,but is portfast feature is enabled then the port goes up on immediate next second by avoiding listening and  learning ( 15+15 sec) steps 

Note: 

1. TCN stands for Topology change notification , it is the message which have the information about the change in topology by failure of active any link
2. Slow transition i.e in case of alternate link fails it taks 50 sec to recover :
   Blocking (20s) + Listening (15s) + Learning (15s) =>Forwarding

UplinkFast:

• UplinkFast is a Cisco specific feature and nowadays it is not being used.
• It also improves the convergence time of STP in the event of the failure of active link. 
• UplinkFast is recommended to be enabled only for switches with blocked ports,

Backbonefast:

In normal condition if indirect link goes down it takes about 50 sec to up the alternate link , but if backbone fast feature is enabled then the indirect link goes up in 30 sec which speedup STP by 20 sec

Port Status in STP (802.1d) and RSTP  (802.1w):

Port states
Disabled Blocking------------Alternate port Listening----Learn MAC address(BPDU Learning----Learn MAC address Forwarding--Learn MAC address	Discarding (replaces disabled, blocking and listening) Learning--------Learn MAC address Forwarding----Learn MAC address
To avoid flapping, it takes 3 seconds for a port to migrate from one protocol to another (STP / RSTP) in a mixed segment.
Port roles
Root (Forwarding) Designated (Forwarding) Non-Designated (Blocking)	Root (Forwarding) Designated (Forwarding) Alternate(Discarding)Backup (Discarding)

 Convergence time of STP Vs RSTP:

As it is been discussed that in normal condition it talk 30  to 50 sec to converse a network , which is long time and has to be compressed, In RSTP convergence is much faster the STP, in case of STP it takes 30 sec (15sec for Listening and 15 sec for Learning) if direct link is failed, and if indirect link fails it takes 50 sec (20 sec for TCN or port role change time (blocking to forwarding)+ 15sec Listening +15sec Learning) and finally in forwarding state while in case of RSTP  the port goes immediately in forwarding state (next sec), 
Note:- Minimum time to converge a network is immediate next second and max time is about 6 second (= 3 hello second).

in case of RSTP the process is goes on the basis of two messages

• Proposal messages 
• Agreement messages

RSTP Configuration:

All the switches in the domain has to be configured with the command in global configuration mode. 

SW-A(config)#spanning-tree mode rapid-pvst

SW-A#show spanning-tree ...............for troubleshooting the port status