Stp Topology and Implement RSTP

The potential effect of a loop in the Layer 2 network is significant. Layer 2 loops could impact connected hosts as well as the network equipment. Layer 2 loops can be prevented by following good design practices and careful implementation of the Spanning Tree Protocol.

Switch#conf t
Switch(config)#hostname D1
D1(config)#spanning-tree mode pvst
D1(config)#banner motd ^ STP Topology Change and RSTP lab ^
D1(config)#line con 0
D1(config-line)#exec-timeout 0 0
D1(config-line)#logging synchronous 
D1(config-line)#exit
D1(config)#int range g1/0/1-3
D1(config-if-range)#switchport mode access 
D1(config-if-range)#switchport trunk
D1(config-if-range)#exit
D1(config)#vlan 2
D1(config-vlan)#name SecondVLAN
D1(config-vlan)#exit
D1(config)#
D1(config)#int vlan 1
D1(config-if)#ip add 10.0.0.1 255.0.0.0
D1(config-if)#no shut

D1(config-if)#
%LINK-5-CHANGED: Interface Vlan1, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to up

D1(config-if)#exit

D1#sh interfaces trunk 
Port        Mode         Encapsulation  Status        Native vlan
Gig1/0/1    on           802.1q         trunking      1
Gig1/0/2    on           802.1q         trunking      1
Gig1/0/3    on           802.1q         trunking      1

Port        Vlans allowed on trunk
Gig1/0/1    1-1005
Gig1/0/2    1-1005
Gig1/0/3    1-1005

Port        Vlans allowed and active in management domain
Gig1/0/1    1,2
Gig1/0/2    1,2
Gig1/0/3    1,2

Port        Vlans in spanning tree forwarding state and not pruned
Gig1/0/1    1,2
Gig1/0/2    1,2
Gig1/0/3    1,2

D1#

Switch#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Switch(config)#hostname D2
D2(config)#banner motd ^ D2 Stp topology change and rstp lab ^
D2(config)#spanning-tree mode pvst 
D2(config)#line con 0
D2(config-line)#exec-timeout 0 0
D2(config-line)#logging synchronous 
D2(config-line)#exit
D2(config)#int range gig1/0/1-3
D2(config-if-range)#switchport mode access 
D2(config-if-range)#switchport mode trunk 

D2(config-if-range)#
%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/1, changed state to down

%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/1, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/2, changed state to down

%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/2, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/3, changed state to down

%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/3, changed state to up

D2(config-if-range)#no shut
D2(config-if-range)#exit
D2(config)#vlan 2
D2(config-vlan)#name SecondVLAN
D2(config-vlan)#exit
D2(config)#interface vlan 1
D2(config-if)#ip add 10.0.0.2 255.0.0.0
D2(config-if)#no shut

D2(config-if)#
%LINK-5-CHANGED: Interface Vlan1, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to up

D2(config-if)#exit

A1#sh version | include MAC
Base ethernet MAC Address       : 00:17:59:A7:51:80
A1#
D1#sh version | include MAC
Base ethernet MAC Address       : 00:0B:BE:41:E6:C6
D1#

D2#sh version | include MAC
Base ethernet MAC Address       : 00:E0:A3:09:23:60
D2#

D1#sh spanning-tree 
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     000B.BE41.E6C6
             This bridge is the root
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     000B.BE41.E6C6
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/1          Desg FWD 4         128.1    P2p
Gi1/0/2          Desg FWD 19        128.2    P2p
Gi1/0/3          Desg FWD 19        128.3    P2p

VLAN0002
  Spanning tree enabled protocol ieee
  Root ID    Priority    32770
             Address     000B.BE41.E6C6
             This bridge is the root
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32770  (priority 32768 sys-id-ext 2)
             Address     000B.BE41.E6C6
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/1          Desg FWD 4         128.1    P2p
Gi1/0/2          Desg FWD 19        128.2    P2p
Gi1/0/3          Desg FWD 19        128.3    P2p

D1#

A1#sh spanning-tree 
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     000B.BE41.E6C6
             Cost        19
             Port        1(FastEthernet0/1)
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     000B.BE79.CD69
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/4            Altn BLK 19        128.4    P2p
Fa0/2            Altn BLK 19        128.2    P2p
Fa0/3            Altn BLK 19        128.3    P2p
Fa0/1            Root FWD 19        128.1    P2p

VLAN0002
  Spanning tree enabled protocol ieee
  Root ID    Priority    32770
             Address     000B.BE41.E6C6
             Cost        19
             Port        1(FastEthernet0/1)
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32770  (priority 32768 sys-id-ext 2)
             Address     000B.BE79.CD69
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/4            Altn BLK 19        128.4    P2p
Fa0/2            Altn BLK 19        128.2    P2p
Fa0/3            Altn BLK 19        128.3    P2p
Fa0/1            Root FWD 19        128.1    P2p

A1#

sh spanning-tree root komutu örnek kullanımı

A1# show spanning-tree root
 Root Hello Max Fwd
Vlan Root ID Cost Time Age Dly Root Port
---------------- -------------------- --------- ----- --- --- ------------
VLAN0001 32769 d8b1.9028.af80 19 2 20 15 Fa0/1
VLAN0002 32770 d8b1.9028.af80 19 2 20 15 Fa0/1


The first thing to look at is the priority value. It is 32768 by default. 
Because we are working with PVST+,
a differentiator is added – the priority value is modified with the extended system ID, which is equal to the
VLAN number. You can see in the output here that our three devices are using default priorities – 32769
for VLAN 1 (32768 + 1) and 32770 for VLAN 2 (32768 + 2). 
For each VLAN, the priority values are the
same for each of the three switches. 
When this happens, the rest of the BID is taken into account. The
rest of the BID includes the base MAC address.
 The lowest base MAC address is used to break the tie