Similarities and Differences between STP and RSTP

A long ago, IEEE developed the STP protocol to remove loops from the network. Later, it updated the STP with the RSTP to fulfill the requirements of the modern network. This tutorial compares STP with RSTP and describes their similarities and differences.

RSTP is the updated version of STP. It uses the same framework STP uses. You can use both in the same network. RSTP features will work in switches that support it, and traditional STP features will work in the switches that support only STP.

Similarities between RSTP and STP

The following are the similarities between STP and RSTP. 

  • Both run a single STP instance on the switch.
  • Both create and use a virtual topology to find switching loops in the network.
  • Both elect a root bridge and use it as the focal point of the virtual topology.
  • Both use the same rules and tiebreakers for the root bridge election.
  • Besides the root bridge, all remaining switches become non-root bridge switches in both.
  • Both select only one root port to reach the root bridge.
  • Both use the same rules and tiebreakers to select the root port.
  • Both select only one designated port for every local segment or end device.
  • Both use the same rules and tiebreakers to select the designated port.
  • After selecting root and designated ports, both block all remaining ports to remove loops.
  • Both STP and RSTP run all ports through a few states. During these states, they learn the network topology, flush CAM table entries, select root and designated ports, and block the ports causing the loops.

With the above similarities, you might wonder why the IEEE bothered to create RSTP in the first place. The following section describes the reasons why IEEE had to create RSTP. 

Differences between RSTP and STP

The main reason for creating RSTP was the convergence. Convergence is the time STP or RSTP takes to react to a network change. STP takes a relatively long time for convergency. With default settings, it takes 50 seconds to reach convergence. RSTP improves it. It reaches convergence within a few seconds (or in slow conditions, in about 10 seconds).

It adds two new port types for rapid convergence. These port types are alternative and backup ports. It uses the alternative port to reach the root bridge if the link connected to the root port fails. It uses the backup port to reach the local segment when the link connected to the designated port fails.

Since it has an alternative or backup port, it does not have to reselect the root or designated ports when a network change occurs. If the root path fails, it enables the alternative port within a few seconds. If the designated path fails, it enables the backup port. It means if the network is running RSTP, it has to wait a maximum of 10 seconds whenever a change occurs.

STP does not have a concept of alternative and designated ports. If the root or designated path fails, it reruns the entire process to select the new root or designated ports. Running the entire process again takes 50 seconds. It means if the network is running STP, it has to wait for 50 seconds whenever a change occurs.

RSTP does not use the learning state. It also renames the blocking state to the discarding state and redefines its use slightly. It combines disabled and blocking states into the discarding state.

In the disabled state, the port can work physically, but it can not forward user frames. It simply meant that the interface was administratively disabled.

The following table compares STP and RSTP port states.

Function STP State RSTP State
The port is administratively disabled Disabled Discarding
The port is not accepting and forwarding user frames Blocking Discarding
The port is not accepting and forwarding user frames but listing BPDUs. Listening Not used
The port is accepting and using user frames to build CAM table entries but not forwarding them. Learning Learning
The port is accepting and forwarding user frames. Forwarding Forwarding

STP and RSTP flush CAM table entries differently. Each CAM table entry has a default age-old time. Once this time has expired, the switch removes the entry from the CAM table. STP waits for this time to expire. RSTP does not wait for this time to expire. It flashes all CAM table entries whenever a change occurs.

ComputerNetworkingNotes CCNA Study Guide Similarities and Differences between STP and RSTP