| 1. | What commands can configure a Catalyst 4500 switch as the Root Bridge on VLAN 10, assuming that the other switches are using the default STP values? |
| Answer: | spanning-tree vlan 10 root primary
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| 2. | Using your Root Bridge answer from question 1, what commands can configure a Catalyst 3550 switch as a secondary or backup Root Bridge on VLAN 10? |
| Answer: | spanning-tree vlan 10 root secondary
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| 3. | Which of the following switches will become the Root Bridge, given the information in the following table? Which switch will become the secondary Root Bridge if the Root Bridge fails? Switch Name | Bridge Priority | MAC Address | Port Costs |
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Catalyst A | 32,768 | 00-d0-10-34-26-a0 | All are 19. | Catalyst B | 32,768 | 00-d0-10-34-24-a0 | All are 4. | Catalyst C | 32,767 | 00-d0-10-34-27-a0 | All are 19. | Catalyst D | 32,769 | 00-d0-10-34-24-a1 | All are 19. |
Questions 4 through 7 are based on a network that contains two switches, Catalyst A and B. Their bridge priorities and MAC addresses are 32,768:0000.aaaa.aaaa and 32,768:0000.bbbb.bbbb, respectively. |
| Answer: | The root bridge will be Catalyst C because its bridge priority has the lowest value. The bridge priority is more significant because it is stored in the upper bits of the Bridge ID field. If Catalyst C fails in its duty as root bridge, Catalyst B will take over as the secondary root bridge. Because Catalyst B has the default bridge priority (32,768), along with another switch, the lowest MAC address will be the deciding factor. Questions 4 through 7 are based on a network that contains two switches, Catalyst A and B. Their bridge priorities and MAC addresses are 32,768:0000.aaaa.aaaa and 32,768:0000.bbbb.bbbb, respectively. |
| 4. | Which switch will become the Root Bridge? |
| Answer: | Catalyst A. The bridge priorities are equal, so the lowest MAC address is the deciding factor. |
| 5. | If switch B's bridge priority is changed to 10,000, which one will be root? |
| Answer: | Catalyst B will become the new root bridge because its new priority is the lowest. |
| 6. | If switch B's bridge priority is changed to 32,769, which one will be root? |
| Answer: | Catalyst A will become the root again because B's priority is slightly higher. |
| 7. | If switch C is introduced with 40000:0000.0000.cccc, which will be the secondary root? |
| Answer: | Catalyst B was previously the secondary root, with the second-highest bridge priority. It will remain the secondary root because Catalyst C has a higher bridge priority. |
| 8. | Suppose that a switch is configured with the spanning-tree vlan 10 root primary command. Then, another switch is connected to the network. The new switch has a bridge priority of 8192. Which one of the following happens? When the new switch advertises itself, the original Root Bridge detects it and lower its bridge priority to 4096 less than the new switch. The new switch becomes and stays the Root Bridge (bridge priority 8192). No change; both switches keep their current bridge priorities. The new switch detects that a Root Bridge already exists and raises its own bridge priority to 32,768.
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| Answer: | b |
| 9. | Three switches in a network have the following bridge priorities: 32,768, 16,384, and 8192. If a fourth switch is configured with spanning-tree vlan 1 root secondary, what is the bridge priority of the switches that become the primary and secondary Root Bridges? |
| Answer: | Primary root: 8192; secondary root: 16,384. (The switch configured with the root secondary keywords can't detect any other potential secondary roots, so it can set its priority only to 28,672.) |
| 10. | What STP timer values automatically can be modified by setting the network diameter? |
| Answer: | Hello timer Forward Delay timer Max Age timer |
| 11. | Which STP timer determines how long a port stays in the Listening state? What is its default value? |
| Answer: | The Forward Delay timer; default 15 seconds |
| 12. | What is the purpose of the Max Age timer? |
| Answer: | It sets the length of time received BPDUs are held if a neighboring switch is not heard from on a nondesignated port. After the Max Age timer expires, the BPDU for the neighbor is flushed and that port enters the Listening state, eventually becoming the new designated port on the segment. |
| 13. | Three switches are connected to each other, forming a triangle shape. STP prevents a loop from forming. What is the most accurate value that could be used for the network diameter? |
| Answer: | 3 |
| 14. | Which of the following will not benefit from STP UplinkFast? An access-layer switch with one uplink port An access-layer switch with two uplink ports An access-layer switch with three uplink ports An access-layer switch with four uplink ports
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| Answer: | a |
| 15. | What command can enable the STP PortFast feature on a switch? What configuration mode must you enter first? |
| Answer: | spanning-tree portfast, in interface-configuration mode |
| 16. | What happens if the STP Hello Time is decreased to 1 second in an effort to speed up STP convergence? What happens if the Hello Time is increased to 10 seconds? |
| Answer: | Setting the Hello timer to 1 second doubles the number of configuration BPDUs that a switch sends, as compared to the default 2-second timer. Although this does share BPDU information more often, it doesn't help the long convergence delay when a port comes up. The significant delays come from the Forward Delay timer, which is used to move a port through the Listening and Learning states. By default, this process takes 30 seconds and is unaffected by the Hello timer. |
| 17. | What switch command safely can adjust the STP timers on the Root Bridge in VLAN 7? Assume that the network consists of Catalysts A, B, and C, all connected to each other in a triangle fashion. For questions 18 and 19, refer to the following output: Switch# show spanning-tree vlan 50 VLAN50 Spanning tree enabled protocol ieee Root ID Priority 8000 Address 00d0.0457.3831 Cost 12 Port 49 (GigabitEthernet0/1) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32818 (priority 32768 sys-id-ext 50) Address 0009.b7ee.9800 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 Interface Designated Name Port ID Prio Cost Sts Cost Bridge ID Port ID ----------------- ------- --- ------ --- ---- -------------------- ------- FastEthernet0/1 128.1 128 19 FWD 12 32818 0009.b7ee.9800 128.1 FastEthernet0/2 128.2 128 19 FWD 12 32818 0009.b7ee.9800 128.2 FastEthernet0/4 128.4 128 100 FWD 12 32818 0009.b7ee.9800 128.4 FastEthernet0/7 128.7 128 19 FWD 12 32818 0009.b7ee.9800 128.7 FastEthernet0/8 128.8 128 19 FWD 12 32818 0009.b7ee.9800 128.8 FastEthernet0/9 128.9 128 19 FWD 12 32818 0009.b7ee.9800 128.9 FastEthernet0/10 128.10 128 19 FWD 12 32818 0009.b7ee.9800 128.10 FastEthernet0/11 128.11 128 19 FWD 12 32818 0009.b7ee.9800 128.11 FastEthernet0/12 128.12 128 19 FWD 12 32818 0009.b7ee.9800 128.12 FastEthernet0/17 128.13 128 19 FWD 12 32818 0009.b7ee.9800 128.13 FastEthernet0/20 128.16 128 19 FWD 12 32818 0009.b7ee.9800 128.16 FastEthernet0/21 128.17 128 19 FWD 12 32818 0009.b7ee.9800 128.17 FastEthernet0/23 128.19 128 19 FWD 12 32818 0009.b7ee.9800 128.19 FastEthernet0/24 128.20 128 19 FWD 12 32818 0009.b7ee.9800 128.20
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| Answer: | Because the three switches form a triangle loop, one link eventually will be placed in the Blocking state. Therefore, the maximum distance across the network is 3 switch hops. This value can be used to define the network diameter to safely adjust the STP timers for faster convergence: spanning-tree vlan 7 root primary diameter 3
For questions 18 and 19, refer to the following output: Switch# show spanning-tree vlan 50 VLAN50 Spanning tree enabled protocol ieee Root ID Priority 8000 Address 00d0.0457.3831 Cost 12 Port 49 (GigabitEthernet0/1) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32818 (priority 32768 sys-id-ext 50) Address 0009.b7ee.9800 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 Interface Designated Name Port ID Prio Cost Sts Cost Bridge ID Port ID ----------------- ------- --- ------ --- ---- -------------------- ------- FastEthernet0/1 128.1 128 19 FWD 12 32818 0009.b7ee.9800 128.1 FastEthernet0/2 128.2 128 19 FWD 12 32818 0009.b7ee.9800 128.2 FastEthernet0/4 128.4 128 100 FWD 12 32818 0009.b7ee.9800 128.4 FastEthernet0/7 128.7 128 19 FWD 12 32818 0009.b7ee.9800 128.7 FastEthernet0/8 128.8 128 19 FWD 12 32818 0009.b7ee.9800 128.8 FastEthernet0/9 128.9 128 19 FWD 12 32818 0009.b7ee.9800 128.9 FastEthernet0/10 128.10 128 19 FWD 12 32818 0009.b7ee.9800 128.10 FastEthernet0/11 128.11 128 19 FWD 12 32818 0009.b7ee.9800 128.11 FastEthernet0/12 128.12 128 19 FWD 12 32818 0009.b7ee.9800 128.12 FastEthernet0/17 128.13 128 19 FWD 12 32818 0009.b7ee.9800 128.13 FastEthernet0/20 128.16 128 19 FWD 12 32818 0009.b7ee.9800 128.16 FastEthernet0/21 128.17 128 19 FWD 12 32818 0009.b7ee.9800 128.17 FastEthernet0/23 128.19 128 19 FWD 12 32818 0009.b7ee.9800 128.19 FastEthernet0/24 128.20 128 19 FWD 12 32818 0009.b7ee.9800 128.20
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| 18. | What is the Bridge ID for the current Root Bridge? Is the switch that produced this output the actual Root Bridge? |
| Answer: | The root Bridge ID is 8000:00d0.0457.3831. The local switch is not the root bridge because its Bridge ID (32818:0009.b7ee.9800) is different from the root. |
| 19. | What is the path cost of interface FastEthernet 0/4, and why is it different from the others? |
| Answer: | The path cost is 100. This is because that interface currently is operating at 10Mbps (STP cost 100), whereas the others are operating at 100Mbps (STP cost 19). |
| 20. | Why does the column marked "Designated Bridge ID" have the same value for every switch port? |
| Answer: | Each of the switch ports shown has won the election to become the designated port for its local segment. Each designated port must identify its own Bridge ID, which is 32818:0009.b7ee.9800. Naturally, the Bridge ID is the same for all ports on VLAN 50. |
| 21. | Suppose that you need to troubleshoot your spanning-tree topology and operation. What commands and information can you use on a switch to find information about the current STP topology in VLAN 39? |
| Answer: | The show spanning-tree vlan 39 root command displays the current root bridge and the root port for VLAN 39. The show spanning-tree vlan 39 brief command shows a listing of every switch port on VLAN 39, along with its path cost and STP state. The designated Bridge ID is also shown on every switch port segment. For a quick summary of the total number of ports participating in each active VLAN, use the show spanning-tree summary command. |
