Rapid Spanning Tree Protocol (RSTP)

Routing management is an important component of networking. Apart from routing packets from source to destination it involves complex management of packets to maintain performance of the network. The protocols which monitor performance of the network and work on detection and removal of redundant links are equally important at layer 2 operations. It tends to solve issues of network performance degradations when systems use shared telecommunication channels on local area networks. 

Today we look more in detail about RSTP – Rapid Spanning tree protocol, its features and how it is configured in a network. 

What is RSTP-Rapid Spanning Tree Protocol?

Rapid Spanning Tree Protocol is a fast convergent and advanced version of STP or Spanning Tree protocol. It is used to prevent loops in bridges and broadcast storms in LANs redundant networks. It achieves faster convergence by bypassing the blocking and listening state of STP and permitting only one active path between two devices. Disabled connections are used as backup / redundant paths to support active connection in event of failure. The IEEE standard for Rapid Spanning Tree protocol is 802.1w. 

Commonalities between STP and RSTP 

• Lowest bridge ID is elected as root bridge in both STP and RSTP connection
• In STP and RSTP BPDUs are forwarded between switches
• Both in STP and RSTP root and designated ports are elected in similar manner

The Spanning Tree Protocol (STP) and Rapid Spanning Tree Protocol (RSTP) are distinguished by their capability to prevent loops in network topologies containing redundant links. While the STP was initially developed to address this issue, it has certain limitations, including a lengthy convergence time and an inability to adapt swiftly to changes in network topology. The RSTP, on the other hand, overcomes these drawbacks and provides a more efficient solution for loop prevention while being responsive to dynamic alterations in the network setup.

RSTP Port Roles

RSTP has four port roles as under:

• Root port is best path port to forward data to bridge.
• Designated port is a non-root port used as forwarded port for each LAN segment.
• Backup port is backup path to segment to which another bridge port connection is established. 
• Alternate port is a backup port having less desirable cost of path. They remain in block state.

RSTP supports three port states as under: 

• Discarding – No data transport happens in this state
• Learning – Ports learn about MAC address but no frames are forwarded
• Forwarding – ports send data and are in full operational state 

Features of RSTP

• Network loops prevention
• Prevention of redundancy
• It provides faster convergence
• Backward compatible with STP
• No artificial forward delay timers required 

Rapid Spanning Tree Protocol: Configuration

Step 1: To configure RSTP – Rapid Spanning Tree protocol it needs to be enabled.

s1# configure terminal

Provide configuration command, one per line, End with CNTL/Z.

s1(config)#spanning-tree mode rapid-pvst 

s1(config)#interface fa0/2 

s1(config)#spanning-tree link-type point-to-point

s1(config)#end

s1(config)#clear spanning-tree detected protocols

s1(config)#

To check if switch runs RSTP use below command 

S1(config)#show spanning-tree 

Step 2: Configure S1 as the ROOT bridge for VLAN1 and 5 

 s1# configure terminal

Provide configuration command, one per line, End with CNTL/Z.

s1(config)#spanning-tree vlan 1 root primary

s1(config)#spanning-tree vlan 5 root primary

s1(config)#end

%SYS-5-CONFIG_I: Configured from console by console

s1#

s1(config)#Show spanning-tree vlan 1 

VLAN001 

Spanning tree enabled protocol rstp

  Root ID    Priority          24577

              Address         0014.f2d2.4180

              Cost             9

              Port              216 (Port-channel21)

              Hello Time   3 sec  Max Age 15 sec  Forward Delay 12 sec

  Bridge ID  Priority         32766  (priority 32765 sys-id-ext 1)

  Address         001c.57d8.9000

Hello Time   3 sec  Max Age 15 sec  Forward Delay 12 sec

Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type

——————- —- — ——— ——– —————————

Po21                Root FWD 9         128.216  P2p 

Po23                Altn BLK 9         128.232  P2p

s1(config)#Show spanning-tree vlan 5

VLAN005

  Spanning tree enabled protocol rstp

  Root ID   Priority          24586

              Address          0014.f2d2.4180

              Cost              9

              Port                216 (Port-channel21)

            Hello Time   2 sec  Max Age 15 sec  Forward Delay 12 sec

  Bridge ID  Priority          32777 (priority 32767 sys-id-ext 10)

              Address         001c.57d8.9000

             Hello Time   2 sec  Max Age 15 sec  Forward Delay 12 sec

              Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type

——————- —- — ——— ——– —————————

Po21                Root FWD 9         128.216  P2p 

Po23                Altn BLK 9         128.232  P2p

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ABOUT THE AUTHOR

Rashmi Bhardwaj

Founder of AAR TECHNOSOLUTIONS, Rashmi is an evangelist for IT and technology. With more than 12 years in the IT ecosystem, she has been supporting multi domain functions across IT & consultancy services, in addition to Technical content making.

You can learn more about her on her linkedin profile – Rashmi Bhardwaj