What is RSTP (Rapid Spanning Tree Protocol)

RSTP (Rapid Spanning Tree Protocol) is like an intelligent helper for the network. It stops loops and works faster than the older STP. This protocol makes sure devices adjust quickly to changes in the LAN.

It keeps things running well, making sure everything is available and works smoothly. That’s why many experts like using it. In this article, we’ll check out what RSTP is, its excellent features, and how it works in networking.

The Standard Spanning Tree Protocol prevents network loops on Cisco Switches (802.1D). If there’s a link or cable problem, it figures out the best route for the packet to reach its destination.

Rapid Spanning Tree Definition and Features

What Exactly is RSTP (Rapid Spanning Tree Protocol) in Networking, and How Does It Work?

The classical or Standard Spanning Tree Protocol is 50 seconds in total for the network to maintain its continuity. Classic STP Port States are different from the Rapid STP Protocol.

Furthermore, Standard STP has Blocking, Listening, Learning, and Forwarding port states. In short, it has BackboneFast, UplinkFast, and PortFast for quick transfer to keep network flow.

When configuring the original STP protocol, we also enable the PortFast, UplinkFast, and BackboneFast features for fast Convergence processing.

After enabling these features, the STP Convergence duration will be 30 seconds, which is the result of an indirect fault on the network.

In today’s computer networks, it is essential to carry high traffic, such as audio and video, on the network without interruption.

RSTP has been developed instead since an extensive network will also be useless on the Original STP Protocol on Cisco Switches. The most significant advantage of this protocol is that it is not specific to a brand.

Rapid ST Protocol (802.1W) provides higher speed reconfiguration (Convergence) than the Original STP Protocol. While the standard STP (802.1D) protocol requires a maximum of 50 seconds for network recalculation, this protocol does this in 1 second.

The Port States of Rapid Spanning Tree

The Rapid Spanning Tree protocol covers all features, such as PortFast, UplinkFast, and BackboneFast.

Also, older Switches that work on an existing network structure and use the standard STP protocol can provide compatibility with RSTP-supported Switches.

1. Discarding Port State

The discarding port state of RSTP is a new port state according to STP, and it includes the blocks and listening ports in STP.

2. Learning Port State

Learning port status works as in STP. So, it has an active role in topology and can be activated immediately. It also knows the MAC addresses on the network.

3. Forwarding Port State

The forwarding port state is also active in the topology and learns the MAC addresses.

Other Port States of RSTP

In addition to the above RSTP Port states, two new port states have been added. The primary purpose of these ports is to speed up the STP Convergence process. These;

1. Alternate Port State

When there is a link failure on a Cisco switch or there is no target route to the Root Bridge, the Switch immediately triggers the Alternate port without initiating the STP Convergence process to find an alternate route. In short, Switch evaluates the Alternate port state as plan B.

2. Backup Port State

When a Designated port in the Rapid Spanning Tree is faulty, it immediately activates a Backup port as Designated. A Backup port status, such as the alternate port status, immediately switches on without STP Convergence operation.

When you examine the image on the network, you can better understand the Backup Port logic. If you remove the HUB device between IOU2 and IOU3, the IOU2 Ethernet 3/3 and Ethernet 0/2 interfaces will be set to Designated.

GNS3 Network Topology

IOU1#show running-config | inc spanning-tree
spanning-tree mode rapid-pvst
spanning-tree extend system-id
IOU1#

IOU2#show running-config | inc spanning-tree
spanning-tree mode rapid-pvst
spanning-tree extend system-id
IOU2#

IOU3#show running-config | inc spanning-tree
spanning-tree mode rapid-pvst
spanning-tree extend system-id
IOU3#

When we apply the show spanning-tree command on IOU2 and IOU3 Switches in the topology above, you can check the following output …

IOU2#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32769
             Address     aabb.cc00.0100
             Cost        100
             Port        2 (Ethernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0200
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Desg FWD 100       128.1    Shr 
Et0/1               Root FWD 100       128.2    Shr 
Et0/2               Desg FWD 100       128.3    Shr 
Et0/3               Desg FWD 100       128.4    Shr 
Et1/0               Desg FWD 100       128.5    Shr 
Et1/1               Desg FWD 100       128.6    Shr 
Et1/2               Desg FWD 100       128.7    Shr 
Et1/3               Desg FWD 100       128.8    Shr 
Et2/0               Desg FWD 100       128.9    Shr 
Et2/1               Desg FWD 100       128.10   Shr 
Et2/2               Desg FWD 100       128.11   Shr 
Et2/3               Desg FWD 100       128.12   Shr 
Et3/0               Desg FWD 100       128.13   Shr 
Et3/1               Desg FWD 100       128.14   Shr 
Et3/2               Desg FWD 100       128.15   Shr 
Et3/3               Back BLK 100       128.16   Shr 
          
          
IOU2#

IOU3 Switch shows spanning-tree command output;

IOU3#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32769
             Address     aabb.cc00.0100
             Cost        100
             Port        11 (Ethernet2/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0300
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Desg FWD 100       128.1    Shr 
Et0/1               Desg FWD 100       128.2    Shr 
Et0/2               Desg FWD 100       128.3    Shr 
Et0/3               Altn BLK 100       128.4    Shr 
Et1/0               Desg FWD 100       128.5    Shr 
Et1/1               Desg FWD 100       128.6    Shr 
Et1/2               Desg FWD 100       128.7    Shr 
Et1/3               Desg FWD 100       128.8    Shr 
Et2/0               Desg FWD 100       128.9    Shr 
Et2/1               Desg FWD 100       128.10   Shr 
Et2/2               Root FWD 100       128.11   Shr 
Et2/3               Desg FWD 100       128.12   Shr 
Et3/0               Desg FWD 100       128.13   Shr 
Et3/1               Desg FWD 100       128.14   Shr 
Et3/2               Altn BLK 100       128.15   Shr 
Et3/3               Desg FWD 100       128.16   Shr 
          
          
IOU3#

Comparison

Rapid STP is a faster protocol than the old version. The purpose of both protocols is to maintain the continuity of the network by preventing loops. The slowness of the STP protocol is due to the Max-Age Timer (20 Seconds) and Forward Delay Timer (15 Seconds) periods, and you can change these values if you wish.

After configuring BackboneFast on Cisco Switches, we can save 20 seconds by skipping the Max-Age Timer. The classic STP network recomputation reduces from 50 seconds to 30 seconds.

Even if we change some settings to improve STP performance, the Rapid Spanning Tree Protocol will perform the recalculation process in less than 1 second.

How Does the RSTP Protocol Work?

In the original STP (802.1D) protocol, BPDU packets are sent only by the Root Bridge Switch. If the RSTP BPDU is in a packet exchange, each Switch will send its BPDU packets in one second (Hello Time).

You can think of it as the RSTP protocol, such as OSPF or EIGRP Routing Protocol. RSTP uses a mechanism that is very different from the original STP.

In short, it uses a keepalive mechanism to keep the network topology alive.

In a network using Rapid Spanning Tree Protocol, a topology change is not considered as a result of a link error. This is regarded as a topology change in the Classic Spanning Tree Protocol. Also, it sends Multicast Frames to update the MAC addresses of all Switches.

The RSTP working logic is as follows:

When a topology change occurs on RSTP-enabled Switches, a Topology Change Timer with twice the Hello Time will start. TC Timer will make for all non-edge Designated and Root ports.
All of the MAC addresses learned through these ports will be cleared.
As soon as the TC Timer activates, the Topology Change bit value will be set in the BPDU packet to send from the non-Edge and Root ports.
A neighboring switch that receives a TC-valued BPDU packet will clear the MAC addresses of all other interfaces except for the interface that it received.
Then, the neighboring Switch itself will initiate a Topology Change and set the TC Bit value. Finally, these packets will be sent to all the Designated and Root ports.

Frequently Asked Questions About RSTP

Why is RSTP so much faster than classic Spanning Tree?

Two magic words: Alternate and Backup. Classic STP recalculates the whole topology from scratch when a failure happens. The network freezes for 50 seconds.

This fast protocol, on the other hand, picks backup paths in advance. When a cable breaks, the Alternate port takes over in milliseconds. It does not waste time solving puzzles.

What is more, every switch constantly sends BPDUs. It forms neighborships like OSPF. It detects the cut within three hello intervals. So voice or video traffic does not feel it.

Besides, port states are also simpler. Discarding replaced Blocking and Listening. Unneeded transitions are gone. As a result, recovery takes less than a second.

How do Alternate and Backup ports speed up network recovery?

The Alternate port waits ready as the backup path to the root bridge. When the main link breaks, the Switch does no calculation. This backup path becomes Forwarding right away.

The Backup port is the spare of the designated port on the same segment. If the designated port fails in a hub setup, the backup immediately takes its role. Again zero waiting time.

This way, network recovery drops below a second. Unlike classic STP, both ports do not actively listen. They only start duty at the moment of the event.

For example, suppose the main fiber port on a switch breaks. The Alternate port takes over right away. Users do not feel the cut. Also, redundancy is almost an invisible superhero.

What exactly does the Discarding port state do?

The Discarding port state gathers old STP’s Blocking and Listening under one roof. The port does not forward data but keeps listening to BPDUs.

This way no loop forms in the network but the switch keeps watching the topology. It can quickly switch to Learning if needed. Actually, it is more of a strategic wait than just waiting.

Also, this state prevents wasting time by switching between two states for no reason. Because every state change eats milliseconds. This protocol spends time like gold.

In short, the switch does not block the port for nothing. It listens and gets ready for the fastest switch. No network admin action is needed.

How did the BPDU mechanism change in the new generation spanning tree?

In classic STP, only the root bridge produced BPDUs, the others just forwarded them. Now each switch sends its own BPDU at the Hello interval. This works like a keepalive.

If a neighbor does not get a BPDU for three Hellos, it understands the link is broken. It does not wait for the root’s decision for a topology change. It reacts on its own right away.

So this mechanism takes the network out of being active-passive. Every switch also becomes a watchman. The base of a safe and fast network is formed.

In addition, the TC (Topology Change) process is also smarter. The switch only cleans the MAC addresses of the affected ports. No multicast frames flood the whole network. Efficiency is at the top level.

Why is this protocol a must for real-time traffic like voice and video?

A 50-second cut in a VoIP call drops the call. The video meeting freezes. Classic STP can only recover in that time.

The Rapid version, on the other hand, does not let you feel the line cut. The backup path takes over right away. The user does not even hear a momentary crackle.

Even if there are frequent changes on the network, classic STP constantly recalculates. This also leads to packet loss. This fast protocol only cleans the MAC table of the affected ports.

What is more, thanks to Edge ports (PortFast), end user connections become Forwarding right away. You get on the network as soon as you plug in your computer. It works instantly.

Is there a compatibility problem between an old switch and Rapid Spanning Tree?

There is no problem at all. The standard works fully backward compatible with old STP (802.1D). The old switch only understands classic BPDUs.

The new switch detects this and switches to old mode just for that port. It keeps running fast on its other ports. This way the switch protects the network as a whole.

But recovery time gets longer on segments that work together with old devices. Still, it is better than before. You get full performance if you fully replace them over time.

So you can manage the transition process without pain. Refresh the edge switches first and enjoy the speed. You can leave the core for later.

Conclusion

RSTP is active by default on most Switches used today, and they run smoothly without the need for any configuration.

What is RSTP (Rapid Spanning Tree Protocol – 802.1W)?