Spanning-Tree Customization - Port Cost & Port Priority

We are going to review how to modify STA(Spanning Tree Algorithm) selection. We are going to use figure 1 for this.

Figure 1

Keep the following STP Rules in mind that will be used to choose a path:

1. Lowest Bridge ID -
2. Lowest root path cost
3. Lowest sender bridge ID
4. Lowest sender port ID

Based on the below output we can see that the root port for vlan 25 is Gi1/47. Let supposed we wanted to use Gi1/48 instead of Gi1/47. How would we do this? We would need to modify the port cost on Switch 02 or modify the port priority that we are receiving from 01.  In our case we will modify the port cost on Switch 02 for Gi1/48.  The port cost is related to the port bandwidth. If you have a 10Mbit port then the port cost will be higher. Its an inverse affect.


Cisco_4948E_02#show spanning-tree vlan 25

VLAN0025
  Spanning tree enabled protocol rstp
  Root ID    Priority    32793
             Address     4055.39a7.bb80
             Cost        4
             Port        47 (GigabitEthernet1/47)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32793  (priority 32768 sys-id-ext 25)
             Address     4055.39a8.1000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/47              Root FWD 4         128.47   P2p
Gi1/48              Altn BLK 4         128.48   P2p


Lets make our changes and confirm Gi1/48 is now being used as the root port.

Cisco_4948E_02(config)#int gi 1/48
Cisco_4948E_02(config-if)#spanning-tree vlan 25 cost 2
Cisco_4948E_02(config-if)#end

Cisco_4948E_02#show spanning-tree vlan 25

VLAN0025
  Spanning tree enabled protocol rstp
  Root ID    Priority    32793
             Address     4055.39a7.bb80
             Cost        2
             Port        48 (GigabitEthernet1/48)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32793  (priority 32768 sys-id-ext 25)
             Address     4055.39a8.1000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/47              Altn BLK 4         128.47   P2p
Gi1/48              Root FWD 2         128.48   P2p

Now lets put everything back to default and we will modify the port-priority on Switch 01 and see how it affects the decision Switch 02 makes. Based on the below output all is back to Normal.

Cisco_4948E_02#show spanning-tree vlan 25

VLAN0025
  Spanning tree enabled protocol rstp
  Root ID    Priority    32793
             Address     4055.39a7.bb80
             Cost        4
             Port        47 (GigabitEthernet1/47)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32793  (priority 32768 sys-id-ext 25)
             Address     4055.39a8.1000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/47              Root FWD 4         128.47   P2p
Gi1/48              Altn BLK 4         128.48   P2p

Lets make some changes on Switch 01, but before we do this lets verify some details to compare afterwards. We can see the port id being received from Switch 01 is 128.47 for Gi1/47 and 128.48 for Gi1/48. We will modify this on Switch 01 and by changing this we will prefer Gi1/48. This will keep us inline with the original goal of using Gi1/48. 

Cisco_4948E_02#show spanning-tree vlan 25 detail

 VLAN0025 is executing the rstp compatible Spanning Tree protocol
  Bridge Identifier has priority 32768, sysid 25, address 4055.39a8.1000
  Configured hello time 2, max age 20, forward delay 15, transmit hold-count 6
  Current root has priority 32793, address 4055.39a7.bb80
  Root port is 47 (GigabitEthernet1/47), cost of root path is 4
  Topology change flag not set, detected flag not set
  Number of topology changes 10 last change occurred 00:32:15 ago
          from GigabitEthernet1/47
  Times:  hold 1, topology change 35, notification 2
          hello 2, max age 20, forward delay 15
  Timers: hello 0, topology change 0, notification 0, aging 300

 Port 47 (GigabitEthernet1/47) of VLAN0025 is root forwarding
   Port path cost 4, Port priority 128, Port Identifier 128.47.
   Designated root has priority 32793, address 4055.39a7.bb80
   Designated bridge has priority 32793, address 4055.39a7.bb80
   Designated port id is 128.47, designated path cost 0
   Timers: message age 16, forward delay 0, hold 0
   Number of transitions to forwarding state: 3
   Link type is point-to-point by default
   BPDU: sent 10, received 1362

 Port 48 (GigabitEthernet1/48) of VLAN0025 is alternate blocking
   Port path cost 4, Port priority 128, Port Identifier 128.48.
   Designated root has priority 32793, address 4055.39a7.bb80
   Designated bridge has priority 32793, address 4055.39a7.bb80
   Designated port id is 128.48, designated path cost 0
   Timers: message age 16, forward delay 0, hold 0
   Number of transitions to forwarding state: 4
   Link type is point-to-point by default
   BPDU: sent 14, received 88569


Cisco_4948E_01(config)#int gi 1/47
Cisco_4948E_01(config-if)#spanning-tree port-priority 32
Cisco_4948E_01(config-if)#int gi 1/48
Cisco_4948E_01(config-if)#spanning-tree port-priority 16
Cisco_4948E_01(config-if)#endCisco_4948E_02#show spanning-tree vlan 25 detail

 VLAN0025 is executing the rstp compatible Spanning Tree protocol
  Bridge Identifier has priority 32768, sysid 25, address 4055.39a8.1000
  Configured hello time 2, max age 20, forward delay 15, transmit hold-count 6
  Current root has priority 32793, address 4055.39a7.bb80
  Root port is 48 (GigabitEthernet1/48), cost of root path is 4
  Topology change flag set, detected flag not set
  Number of topology changes 11 last change occurred 00:00:02 ago
          from GigabitEthernet1/48
  Times:  hold 1, topology change 35, notification 2
          hello 2, max age 20, forward delay 15
  Timers: hello 0, topology change 32, notification 0, aging 300

 Port 47 (GigabitEthernet1/47) of VLAN0025 is alternate blocking
   Port path cost 4, Port priority 128, Port Identifier 128.47.
   Designated root has priority 32793, address 4055.39a7.bb80
   Designated bridge has priority 32793, address 4055.39a7.bb80
   Designated port id is 32.47, designated path cost 0
   Timers: message age 15, forward delay 0, hold 0
   Number of transitions to forwarding state: 3
   Link type is point-to-point by default
   BPDU: sent 10, received 1400

 Port 48 (GigabitEthernet1/48) of VLAN0025 is root forwarding
   Port path cost 4, Port priority 128, Port Identifier 128.48.
   Designated root has priority 32793, address 4055.39a7.bb80
   Designated bridge has priority 32793, address 4055.39a7.bb80
   Designated port id is 16.48, designated path cost 0
   Timers: message age 15, forward delay 0, hold 0
   Number of transitions to forwarding state: 5
   Link type is point-to-point by default
   BPDU: sent 16, received 88606

Cisco_4948E_02#
 
 Cisco_4948E_02#show spanning-tree vlan 25

VLAN0025
  Spanning tree enabled protocol rstp
  Root ID    Priority    32793
             Address     4055.39a7.bb80
             Cost        4
             Port        48 (GigabitEthernet1/48)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32793  (priority 32768 sys-id-ext 25)
             Address     4055.39a8.1000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/47              Altn BLK 4         128.47   P2p
Gi1/48              Root FWD 4         128.48   P2p