Spanning tree, PVST lab - GNS3

as usual, i'm using cisco 3740 with additional slot NM-16ESW.
for the 1st stage, we only use one link for every connection between every switch (i'll refer the router as the switch for the rest of the tutorial)


configure on D1, port fast 1/1 and 1/7
 no shutdown
 switchport trunk encap dot1q
 switchport mode trunk


on D2, no span tree yet on D2 because all port still down
 D2(config)#do sh span br
 No spanning tree instances exist.


configure the same we did on D2 for port 1/1 and 1/3
immediately after port change to trunk mode, you'll notice that it is in the listenin(LIS) mode
this is when they running the algorithm to find a loop
later the status will change to (learn)LRN then (forwarding)FWD
 D2(config-if)#do sh sp br
  ...




  Name                 Port ID Prio Cost  Sts Cost  Bridge ID            Port ID
  -------------------- ------- ---- ----- --- ----- -------------------- -------
  FastEthernet1/1      128.42   128    19 LIS     0 32768 c404.0f45.0000 128.42

apply the same configuration for the A2 and A1

 on A2, fast 1/5 and 1/3

 on A1, fast 1/5 and 1/7 (once this int is up, then looping will happen)

there will be no blocked port until you configure the fast 1/7, where the looping start to exist

right after fast 1/7 up, spanning tree will find and choose which port need to be blocked

now show the spanning tree on each switch

D1#show spanning-tree br

VLAN1

  Spanning tree enabled protocol ieee

  Root ID    Priority    32768

             Address     c404.0f45.0000

             This bridge is the root //this switch was elected as root

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32768

             Address     c404.0f45.0000

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time 0

Interface                                   Designated

Name                 Port ID Prio Cost  Sts Cost  Bridge ID            Port ID

-------------------- ------- ---- ----- --- ----- -------------------- -------

FastEthernet1/1      128.42   128    19 FWD     0 32768 c404.0f45.0000 128.42

FastEthernet1/7      128.48   128    19 FWD     0 32768 c404.0f45.0000 128.48

D2#show span br

VLAN1

  Spanning tree enabled protocol ieee

  Root ID    Priority    32768

             Address     c404.0f45.0000

             Cost        19

             Port        42 (FastEthernet1/1)

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32768

             Address     c405.0f45.0000

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time 0

Interface                                   Designated

Name                 Port ID Prio Cost  Sts Cost  Bridge ID            Port ID

-------------------- ------- ---- ----- --- ----- -------------------- -------

FastEthernet1/1      128.42   128    19 FWD     0 32768 c404.0f45.0000 128.42

FastEthernet1/3      128.44   128    19 FWD    19 32768 c405.0f45.0000 128.44

A2#sh span br

VLAN1

  Spanning tree enabled protocol ieee

  Root ID    Priority    32768

             Address     c404.0f45.0000

             Cost        38

             Port        44 (FastEthernet1/3)

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32768

             Address     c407.0f45.0000

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time 0

Interface                                   Designated

Name                 Port ID Prio Cost  Sts Cost  Bridge ID            Port ID

-------------------- ------- ---- ----- --- ----- -------------------- -------

FastEthernet1/3      128.44   128    19 FWD    19 32768 c405.0f45.0000 128.44

FastEthernet1/5      128.46   128    19 BLK    19 32768 c406.0f45.0000 128.46 //blocked port

A1#show spanning-tree br

VLAN1

  Spanning tree enabled protocol ieee

  Root ID    Priority    32768

             Address     c404.0f45.0000

             Cost        19

             Port        48 (FastEthernet1/7)

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32768

             Address     c406.0f45.0000

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time 0

Interface                                   Designated

Name                 Port ID Prio Cost  Sts Cost  Bridge ID            Port ID

-------------------- ------- ---- ----- --- ----- -------------------- -------

FastEthernet1/5      128.46   128    19 FWD    19 32768 c406.0f45.0000 128.46

FastEthernet1/7      128.48   128    19 FWD     0 32768 c404.0f45.0000 128.48

by default, switch with the lowest MAC address will be chosen as the root, to change this, try apply spanning-tree vlan 1 root primary on D2

D2(config)#spanning-tree vlan 1 root primary

 VLAN 1 bridge priority set to 8192

 VLAN 1 bridge max aging time unchanged at 20

 VLAN 1 bridge hello time unchanged at 2

 VLAN 1 bridge forward delay unchanged at 15

now check again, D2 should be the root now.

D2#show spanning-tree bri

VLAN1

  Spanning tree enabled protocol ieee

  Root ID    Priority    8192

             Address     c405.0f45.0000

             This bridge is the root

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec ....

if you have multiple VLAN, you can chose which switch to be the root by applying the same command, just change the VLAN number

let's try this out, first, configure VTP for easy management on VLAN

D1(vlan)#vtp domain LAB

Domain name already set to LAB .

D1(vlan)#vtp server

Setting device to VTP SERVER mode

do the same for all the other 3 switches, just the change to CLIENT mode

then, create 1 VLAN on VTP sever (D1)

D1(vlan)#vlan 100 name seratus

VLAN 100 added:

    Name: seratus

make sure all other switches have the same vlan database

A1#show vlan-switch

VLAN Name                             Status    Ports

---- -------------------------------- --------- -------------------------------

1    default                          active    Fa1/0, Fa1/1, Fa1/2, Fa1/3

                                                Fa1/4, Fa1/6, Fa1/8, Fa1/9

                                                Fa1/10, Fa1/11, Fa1/12, Fa1/13

                                                Fa1/14, Fa1/15

100  seratus                          active

1002 fddi-default                     active

1003 token-ring-default               active

1004 fddinet-default                  active

1005 trnet-default                    active

by default calculation, D1 will be the root switch

D1#show spanning-tree br

....

VLAN100

  Spanning tree enabled protocol ieee

  Root ID    Priority    32768

             Address     c404.0f45.0001

             This bridge is the root

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32768

             Address     c404.0f45.0001

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time 0

Interface                                   Designated

Name                 Port ID Prio Cost  Sts Cost  Bridge ID            Port ID

-------------------- ------- ---- ----- --- ----- -------------------- -------

FastEthernet1/1      128.42   128    19 FWD     0 32768 c404.0f45.0001 128.42

FastEthernet1/7      128.48   128    19 FWD     0 32768 c404.0f45.0001 128.48

Simply repeat the step above to change the root for specific switch ;)

now bring up all the other port as trunk so each switch have 2 link between them, then you should see more clearly which port being blocked due to redundancy

ah, it is almost 5AM, i'm so lazy to paste all result here, furthermore i got Livepool game to watch later, so i'll just print screen all the result here