Lab 6-2 – STP

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In the network shown in Figure 6-16, 802.1d STP is being used on VLAN 5. The administrator wants to ensure that SwitchA is always the root bridge. They also want to ensure that interface fa0/16 is always the root port on SwitchB and SwitchC. In addition to that, interface fa0/1 on all switches should transition to forwarding as soon as a host connects to it.

Figure 6-16   Lab 6-2

Solution

To ensure that SwitchA is always the root bridge, change the priority on it as shown below:

SwitchA(config)#spanning-tree vlan 5 priority 8192 

To ensure that fa0/16 is always the root port on SwitchB and SwitchC, change the port costs as shown below:

SwitchB(config)#int fa0/16

SwitchB(config-if)#spanning-tree cost 1

SwitchC(config)#int fa0/16

SwitchC(config-if)#spanning-tree cost 1

Finally, to ensure that fa0/1 transitions to forwarding as soon as host connects, enable portfast on these ports as shown below:

SwitchA(config)#int fastEthernet0/1

SwitchA(config-if)#spanning-tree portfast

SwitchB(config)#int fastEthernet0/1

SwitchB(config-if)#spanning-tree portfast

SwitchC(config)#int fastEthernet0/1

SwitchC(config-if)#spanning-tree portfast

To verify that SwitchA is the root bridge, use the show spanning-tree vlan 5 command on SwitchA as shown below:

SwitchA#show spanning-tree vlan 5

VLAN0005

Spanning tree enabled protocol ieee

Root ID    Priority    32773

Address     0013.c3e8.2500

             This bridge is the root

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

–output truncated—

To verify that fa0/16 is the root port, use the show spanning-tree vlan 5 command on SwitchB and SwitchC as shown below:

SwitchB#show spanning-tree vlan 5

–output truncated–

Interface           Role Sts Cost      Prio.Nbr Type

——————- —- — ——— ——– ——————————–

fa0/15              Altn BLK 200         128.17   P2p

fa0/16              Root FWD 119      128.20   P2p

SwitchC#show spanning-tree vlan 5

–output truncated–

Interface           Role Sts Cost      Prio.Nbr Type

——————- —- — ——— ——– ——————————–

fa0/15              Altn BLK 200         128.17   P2p

fa0/16              Root FWD 119      128.20   P2p

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