5.1.9 Packet Tracer - Investigate STP Loop Prevention Answers
Topology
Objectives
In this lab, you will observe spanning-tree port states and watch the spanning-tree convergence process.
- Describe the operation of Spanning Tree Protocol.
- Explain how Spanning Tree Protocol prevents switching loops while allowing redundancy in switched networks.
Background / Scenario
In this activity you will use Packet Tracer to observe the operation of Spanning Tree Protocol in a simple switched network that has redundant paths.
Instructions
Part 1: Observe a Converged Spanning-Tree Instance
Step 1: Verify Connectivity.
Ping from PC1 to PC2 to verify connectivity between the hosts. Your ping should be successful.
C:\> ping 192.168.1.101
Step 2: View spanning-tree status on each switch.
Use the show spanning-tree vlan 1 command to gather information about the spanning tree status of each switch. Complete the table. For the purposes of the activity, only consider information about the Gigabit trunk ports. The Fast Ethernet ports are access ports that have end devices connected and are not part of the interswitch trunk-based spanning tree.
S1# show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.6448.C6E7
Cost 4
Port 26(GigabitEthernet0/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000B.BE31.D3DA
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Desg FWD 19 128.1 P2p
Gi0/2 Root FWD 4 128.26 P2p
Gi0/1 Desg FWD 4 128.25 P2pS2# show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.6448.C6E7
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 0001.6448.C6E7
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Desg FWD 19 128.1 P2p
Gi0/2 Desg FWD 4 128.26 P2p
Gi0/1 Desg FWD 4 128.25 P2pS3# show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.6448.C6E7
Cost 4
Port 25(GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000C.CF45.7534
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/1 Root FWD 4 128.25 P2p
Gi0/2 Altn BLK 4 128.26 P2p| Switch | Port | Status (FWD, BLK…) | Root Bridge? |
|---|---|---|---|
| S1 | G0/1 | FWD | No |
| G0/2 | FWD | No | |
| S2 | G0/1 | FWD | Yes |
| G0/2 | FWD | Yes | |
| S3 | G0/1 | FWD | No |
| G0/2 | BLK | No |
Packet Tracer uses a different link light on one of the connections between the switches.
What do you think this link light means?
It indicates that the port is not forwarding frames because it is in a spanning-tree state, in this case the blocking state.
What path will frames take from PC1 to PC2?
They will go from S1 to S2.
Why do the frames not travel through S3?
The main reason is that spanning tree has placed port G0/2 on S3 in blocking mode. No frames are sent or received on that port.
Why has spanning tree placed a port in blocking state?
If all ports could forward frames, a switching loop would exist in the network. Switching loops can degrade network performance and even cause a network to fail.
Part 2: Observe spanning-tree convergence
Step 1: Remove the connection between S1 and S2.
a. Open a CLI window on switch S3 and issue the command show spanning-tree vlan 1. Leave the CLI window open.
S3# show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.6448.C6E7
Cost 4
Port 25(GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000C.CF45.7534
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/1 Root FWD 4 128.25 P2p
Gi0/2 Altn BLK 4 128.26 P2pb. Select the delete tool from the menu bar and click the cable that connects S1 and S2.
Step 2: Observe spanning-tree convergence.
a. Quickly return to the CLI prompt on switch S3 and issue the show spanning-tree vlan 1 command.
S3# show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.6448.C6E7
Cost 4
Port 25(GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000C.CF45.7534
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/1 Root FWD 4 128.25 P2p
Gi0/2 Desg LIS 4 128.26 P2pb. Use the up-arrow key to recall the show spanning-tree vlan 1 command and issue it repeatedly until the orange link light on the cable turns green. Observe the status of port G0/2.
S3# show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.6448.C6E7
Cost 4
Port 25(GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000C.CF45.7534
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/1 Root FWD 4 128.25 P2p
Gi0/2 Desg FWD 4 128.26 P2pWhat do you see happen to the status of the G0/2 port during this process?
First it was BLK, it then became LIS (listening), then LRN (learning), and finally FWD for forwarding.
You have observed the transition in port status that occurs as a spanning-tree port moves from blocking to forwarding state.
c. Verify Connectivity by pinging from PC1 to PC2. Your ping should be successful.
C:\> ping 192.168.1.101
Are any ports showing an orange link light that indicates that the port is in a spanning-tree state other than forwarding? Why or why not?
No orange link lights are shown because there are no longer redundant paths in the network.
