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Rapid Spanning Tree Protocol and Configurations - RSTP

Today I am going to talk about the widely used protocol in the switching scenarios called as Rapid Spanning tree protocol. RSTP all you know that is a enhanced version of the STP (Spanning Tree Protocol) and is now used in many of the LAN networks there in enterprises.

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RSTP- Rapid Spanning Tree Protocol
Replacing the concept of 5 port states in STP, RSTP comes up with the 3 port states only and these port states are discarding, learning and forwarding. 

Fig 1.1- STP Topology
STP has 5 states and these states are Blocking, Listening, Learning, Forwarding and Disabled where we replaced the blocking, listening and discarding with the discarding state in RSTP. Below is the table showing the states of the STP and RSTP

Fig 1.2- STP and RSTP Port States
Now the question is Why we replaced the 5 port states in STP with 3 port states in RSTP ?
Well there are so many question raised by the various customers where they want to change the convergence time of the STP protocol as STP generally takes 30-50 seconds of convergence time. STP can take 30 to 50 seconds to transit from a blocking state to a forwarding state, RSTP is typically able to respond less than 10 seconds of a physical link failure. 

To reduce the convergence time from one state to another, developers shorten the number of states which leads to decrease in the convergence time to 3-5 sec in the case of failures.

Fig 1.3- RSTP Ports


Now Let's talk about the basic configuration of the Rapid Spanning tree protocol on two switches named as ttlbits-Switch1 and ttlbits-Switch2

Enabling RSTP on ttlbits-Switch1
ttlbits-Switch1>enable
ttlbits-Switch1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
ttlbits-Switch1(config)#spanning-tree mode rapid-pvst
ttlbits-Switch1(config)#end
%SYS-5-CONFIG_I: Configured from console by console
ttlbits-Switch1#show spanning-tree bridge

                                                   Hello  Max  Fwd
Vlan                         Bridge ID              Time  Age  Dly  Protocol
---------------- --------------------------------- -----  ---  ---  --------
VLAN1         32769 (32768,   1) 0014.f2d2.4180    2    20   15  rstp        
VLAN2         32778 (32768,  10) 0014.f2d2.4180    2    20   15  rstp        
VLAN3         32788 (32768,  20) 0014.f2d2.4180    2    20   15  rstp        
VLAN4        32798 (32768,  30) 0014.f2d2.4180    2    20   15  rstp        

ttlbits-Switch1#

Enabling RSTP on ttlbits-Switch2
ttlbits-Switch2>enable
ttlbits-Switch2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
ttlbits-Switch2(config)#spanning-tree mode rapid-pvst
ttlbits-Switch2(config)#end
%SYS-5-CONFIG_I: Configured from console by console
ttlbits-Switch2#show spanning-tree bridge

                                                   Hello  Max  Fwd
Vlan                         Bridge ID              Time  Age  Dly  Protocol
---------------- --------------------------------- -----  ---  ---  --------
VLAN1         32769 (32768,   1) 001c.57d8.9000    2    20   15  rstp        
VLAN2         32778 (32768,  10) 001c.57d8.9000    2    20   15  rstp        
VLAN3         32788 (32768,  20) 001c.57d8.9000    2    20   15  rstp        
VLAN4        32798 (32768,  30) 001c.57d8.9000    2    20   15  rstp        
ttlbits-Switch2#

Configure ttlbits-Switch1 as the ROOT bridge for VLAN 1 and 2
ttlbits-Switch1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
ttlbits-Switch1(config)#spanning-tree vlan 1 root primary
ttlbits-Switch1(config)#spanning-tree vlan 2 root primary
ttlbits-Switch1(config)#end
%SYS-5-CONFIG_I: Configured from console by console
ttlbits-Switch1#

Verify the configurations on Switch2
ttlbits-Switch2#show spanning-tree vlan 1

VLAN1
  Spanning tree enabled protocol rstp
  Root ID    Priority    24577
             Address     0014.f2d2.4180
             Cost        9
             Port        216 (Port-channel21)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     001c.57d8.9000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- ---------------------------
Po20                Root FWD 9         128.216  P2p 
Po30                Altn BLK 9         128.232  P2p 

       
ttlbits-Switch2#show spanning-tree VLAN 2

VLAN2
  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 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32778  (priority 32768 sys-id-ext 10)
             Address     001c.57d8.9000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- ---------------------------
Po20                Root FWD 9         128.216  P2p 
Po30               Altn BLK 9         128.232  P2p 

ttlbits-Switch2#

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