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

Standard Spanning Tree Protocol (802.1D) is a protocol specific to Cisco Switches and was developed to prevent loops that occur on the network. In the link or the cable failure that occurs on the network configured the Spanning Tree Protocol, it is calculated the routes for the packet outgoing to the destination.

What is Rapid Spanning Tree Protocol (RSTP - 802.1W)

Understanding RSTP

The classical or Standard Spanning Tree Protocol is 50 seconds in total for the network to maintain its continuity. Classic STP Port States is different from the Rapid ST Protocol. In addition, Standard STP; has Blocking, Listening, Learning and Forwarding port states and has PortFast, UplinkFast, and BackboneFast features to ensure the continuity of the network.

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 very important to carry high traffic such as audio and video on the network without interruption. RSTP has been developed instead since a large network will also be useless on the Original STP Protocol on Cisco Switches. The biggest 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

The RSTP 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 states of Blocking and Listening ports in STP.

2. Learning Port State

Learning port status works as in STP. Learning port status has an active role in topology and can be activated immediately. It also learns 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.

In addition to the above RSTP Port states, two new port states have been added. The main 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 RSTP is faulty, the RSTP 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 Ethernet3/3 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 show 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 and its purpose in both protocols is to maintain the continuity of the network by preventing loops on the network. 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 It Works?

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 like the RSTP protocol such as OSPF, or EIGRP Routing Protocol. RSTP uses a very different mechanism than 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 considered a topology change in the Classic Spanning Tree Protocol, and also Multicast Frames are sent to update the MAC addresses of all Switches.

The RSTP working logic is as follows:

1. When a topology change occurs on RSTP-enabled Switches, a Topology Change Timer with twice Hello Time will start. TC Timer will make for all non-edge Designated and Root ports.

2. All of MAC addresses learned through these ports will be cleared.

3. As soon as TC Timer activates, the Topology Change bit value will set in the BPDU packet to send from the non-Edge and Root ports.

4. 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.

5. Then the neighboring Switch itself will initiate a Topology Change and set the TC Bit value and finally send these packets over all the Designated and Root ports.

   Final Word


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

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