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Spanning Tree

1. STP port stats

image

2. STP operation

  • Root Bridge: Lowest Bridge ID

    • BID: Priority + MAC address

  • Root ports: (non-root, 1 switch has 1 root port)

    • Lowest Path cost to root bridge (from the port)
    • Lowest Sender BID
    • Lowest Port Priority

  • Designated ports & Non-Designated ports

    • Lowest path cost to the bridge (from the switch)
    • Lowest BID

3. Lab

image

3.1. Default

  • SW1
SW1#show spanning-tree  

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     aabb.cc00.1000
             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     aabb.cc00.1000
             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               Desg FWD 100       128.4    Shr 


SW1#
  • SW2
SW2#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     aabb.cc00.1000
             Cost        100
             Port        3 (Ethernet0/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.2000
             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               Root FWD 100       128.3    Shr 
Et0/3               Desg FWD 100       128.4    Shr 


SW2#
  • SW3
SW3#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     aabb.cc00.1000
             Cost        100
             Port        4 (Ethernet0/3)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.3000
             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               Altn BLK 100       128.2    Shr 
Et0/2               Altn BLK 100       128.3    Shr 
Et0/3               Root FWD 100       128.4    Shr 


SW3#

3.2. Make a switch to be Root Bridge

  • Has 2 methods:
    • Decrease priority that switch (recommend)
    • Command root primary (when all switches are default priority)
SW3(config)#spanning-tree vlan 1 priority ?
  <0-61440>  bridge priority in increments of 4096

SW3(config)#spanning-tree vlan 1 priority 1 ?
  <cr>

SW3(config)#spanning-tree vlan 1 priority 1 
% Bridge Priority must be in increments of 4096.
% Allowed values are: 
  0     4096  8192  12288 16384 20480 24576 28672
  32768 36864 40960 45056 49152 53248 57344 61440
SW3(config)#spanning-tree vlan 1 priority 12288 
SW3(config)#
SW3(config)#
SW3(config)#
SW3(config)#
SW3(config)#do show span
SW3(config)#do show spanning-tree 

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    12289
             Address     aabb.cc00.3000
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    12289  (priority 12288 sys-id-ext 1)
             Address     aabb.cc00.3000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  15  sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Desg FWD 100       128.1    Shr 
Et0/1               Desg LIS 100       128.2    Shr 
Et0/2               Desg LIS 100       128.3    Shr 
Et0/3               Desg FWD 100       128.4    Shr 


SW3(config)#

Second method (priority will be 32678 - 8192 = 24576):

SW3(config)#spanning-tree vlan 1 root ?
  primary    Configure this switch as primary root for this spanning tree
  secondary  Configure switch as secondary root

SW3(config)#spanning-tree vlan 1 root pri
SW3(config)#spanning-tree vlan 1 root primary ?
  diameter  Network diameter of this spanning tree
  <cr>

SW3(config)#spanning-tree vlan 1 root primary 
SW3(config)#
SW3(config)#
SW3(config)#do show spanning-tree             

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     aabb.cc00.3000
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    24577  (priority 24576 sys-id-ext 1)
             Address     aabb.cc00.3000
             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               Desg FWD 100       128.4    Shr 


SW3(config)#

3.3. STP path manipulation

Supose that we will modify SW3 port e0/3 from root port to block port

  • Default:
SW3# show spanning-tree 

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     aabb.cc00.1000
             Cost        100
             Port        4 (Ethernet0/3)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.3000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  15  sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Desg FWD 100       128.1    Shr 
Et0/1               Altn BLK 100       128.2    Shr 
Et0/2               Altn BLK 100       128.3    Shr 
Et0/3               Root FWD 100       128.4    Shr 


SW3#
  • Modify cost of interface
SW3(config-if)#spanning-tree cost 500

SW3#show spanning-tree vlan 1

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     aabb.cc00.1000
             Cost        200
             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.cc00.3000
             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               Altn BLK 100       128.3    Shr 
Et0/3               Altn BLK 500       128.4    Shr 


SW3#

So now SW3, port e0/3 is blocked, e0/1 is root, e0/2 is blocked.

  • Now we want modify port e0/2 to root port, => modify sender port cost (port e0/3 of switch SW2) from 128.4 to 64.
SW2(config)#int e0/3
SW2(config-if)#spanning-tree port-priority 64
SW2# show spanning-tree 

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     aabb.cc00.1000
             Cost        100
             Port        3 (Ethernet0/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.2000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  15  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               Root FWD 100       128.3    Shr 
Et0/3               Desg FWD 100        64.4    Shr 


SW2#

Here result on SW3:

SW3# show spanning-tree 

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     aabb.cc00.1000
             Cost        200
             Port        3 (Ethernet0/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.3000
             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               Altn BLK 100       128.2    Shr 
Et0/2               Root FWD 100       128.3    Shr 
Et0/3               Altn BLK 500       128.4    Shr 


SW3#