RSTP Convergence Improvements
Sections:
Overview:
- Open Standard RSTP and Cisco Proprietary RPVST+ have since made convergence improvements after the release of the original 802.1D Spanning Tree standard
- RSTP has the ability to converge the STP topology in as little as 3 seconds whereas 802.1D would normally take between 30-50 seconds to fully converge
- In this section, lets compare the timers and improvements made between both the 802.1W and original 802.1D standards
802.1D Spanning Tree Timers
In the 802.1D original Spanning Tree standard, there are three important timers that control the behavior and convergence of the protocol. These timers determine how long various states of the protocol last to make decisions about the network topology. Lets analyze the default timers set by the original Spanning Tree standard.
Default 802.1D Timers
- Hello Timer: 2 sec
- MaxAge: 20 sec or 10x the configured Hello timer
- Forward Delay: 15 sec each for Listening & Learning states
- Overall Default Convergence Time: MaxAge + Forward Delay
- = 50 sec for a blocking port to transition to a forwarding state if failover occurs
- It is only in the forwarding state in which a port can transmit client traffic
- It is crucial to ensure all switches in a STP domain utilize the same timers
Available configuration parameters for the different types of Spanning Tree timers.
Displaying the default 802.1D Spanning Tree timers in a show command.
802.1W Spanning Tree Improvements
The introduction of 802.1W Spanning Tree standard brought differences in the timers since the original's release. The release of the 802.1W standard is designed for faster convergence and more efficient network operation. However, some of the timers from the original 802.1D standard are still relevant, though they are handled differently in RSTP.
802.1W Enhancements
- With RSTP, a more dynamic approach is used to achieve faster convergence rather than relying on the 802.1D timers
- Topology Change Notifications:
- In the original 802.1D standard, TCNs need to be directed towards the Root switch to inform all switches when failover occurs to update the STP topology with new port assignments
- In 802.1w, TCNs no longer need to be directed towards the root switch which improves the convergence time of the Spanning Tree domain
- 802.1W utilizes the RSTP Proposal/Agreement Handshake between switches as a synching mechanism to eliminate the need for the 802.1D timers
- The handshake rapidly puts a port into a forwarding/designated state without causing disruption in network services
- 802.1W consists of built-in enhancement features that will be discussed in the later sections
- Uplink-Fast
- Backbone-Fast
