Understanding Rapid Spanning Tree Protocol – 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.
Understanding Rapid Spanning Tree Protocol

Understanding Rapid Spanning Tree Protocol

Understanding Rapid Spanning Tree Protocol

The total time that the Classic or Standard Spanning Tree Protocol will spend to maintain the continuity of the network is 50 secondsClassic STP Port States are different from Rapid Spanning Tree Protocol. Standard STP consists of Blocking, Listening, Learning and Forwarding port status. It also includes PortFast, UplinkFast, and BackboneFast features to ensure network continuity.

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, transporting high traffic such as audio and video seamlessly on the network has a huge amount of precaution. In this type of network, Rapid Spanning Tree Protocol (RSTP) was developed instead of the original STP Protocol on Cisco Switches. In addition, the greatest advantage of the RSTP is that it is not specific to a brand.

Rapid Spanning Tree Protocol (802.1W) provides higher speed convergence than the original STP Protocol. The standard STP (802.1D) protocol requires a maximum of 50 seconds for network recomputation, while the RSTP (802.1W) protocol does this for 1 second.

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Rapid Spanning Tree (RSTP) Port States

The RSTP protocol includes all of the features PortFast, UplinkFast, and BackboneFast. In addition, the old switches on the network can only support the Original Spanning Tree Protocol. Therefore the old Cisco Switches supported the STP protocol adapt to the RSTP protocol. The Rapid Spanning Tree Protocol has three different port states to speed up the recalculation time.

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.

As an example for the Backup Port, you can better understand Rapid Spanning Tree Protocol when you review the image below. If you remove the HUB between IOU2 and IOU3, the IOU2 Ethernet3/3 and Ethernet 0/2 interfaces will be set to Designated.

Understanding Rapid Spanning Tree Protocol


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#

 

Rapid Spanning Tree and Classic Spanning Tree Comparison

The Rapid Spanning Tree Protocol is a faster protocol than the STP Protocol, and the goal in both protocols is to maintain the continuity of the network by preventing looping on the network. The slowness of the STP protocol is caused by the Max Age Timer (20 Seconds) and the Forward Delay Timer (15 Seconds), 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 RSTP 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.

The Rapid Spanning Tree Protocol uses a keepalive mechanism to keep the network topology alive.

In a network using Rapid Spanning Tree Protocol, a link failure is not considered as a topology change. This situation is considered as a topology change in the Classic Spanning Tree Protocol, and it also Multicast Frames sends 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.

Note: Edge port is the port that end-user computers and printer devices connect to. (You can check PortFast Configuration.)

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 sent 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 

Understanding Rapid Spanning Tree Protocol – In this article, we examined Rapid Spanning Tree and what it does in detail. Most switches used today have RSTP enabled by default and work fine without any configuration required. Also, you can better understand Rapid Spanning Tree Protocol by creating and examining the topology for RSTP.

If this article is helpful, send me feedback by commenting! Thanks in advance, take care of yourself!

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