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Chapter 2 499

Chapter 2

“Do I Know This Already?” Quiz

1.Which of the following are the port states when STP has completed convergence?

Answer: A, B

2.Which of the following are transitory port states used only during the process of STP convergence?

Answer: C, D

3.Which of the following bridge IDs would win election as root, assuming that the switches with these bridge IDs were in the same network?

Answer: C

4.Which of the following facts determines how often a root bridge or switch sends a BPDU message?

Answer: B

5.What feature causes an interface to be placed in forwarding state as soon as the interface is physically active?

Answer: E

6.What feature combines multiple parallel Ethernet links between two switches so that traffic is balanced across the links, and so that STP treats all links as one link?

Answer: F

7.What name represents the improved STP standard that lowers convergence time?

Answer: B, D

8.Which of the following RSTP port roles have the same name as a similar role in STP?

Answer: B, D

9.On a 2950 switch, what command lets you change the value of the bridge ID without having to configure a specific value for any part of the bridge ID?

Answer: B

500 Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Questions

10.What command lists spanning-tree status information on 2950 series switches?

Answer: A


1.What routing protocol does a transparent bridge use to learn about Layer 3 address groupings?

Answer: None. Bridges do not use routing protocols. Transparent bridges do not care about Layer 3 address groupings. Devices on either side of a transparent bridge are in the same Layer 3 group—in other words, the same IP subnet.

2.What settings does a bridge or switch examine to determine which should be elected as root of the spanning tree?

Answer: The bridge priority is examined first (the lowest wins). In case of a tie, the lowest bridge ID wins. The priority is prepended to the bridge ID in the actual BPDU message so that the combined fields can be compared easily.

3.If a switch hears three different hello BPDUs from three different neighbors on three different interfaces, and if all three specify that Bridge 1 is the root, how does the switch choose which interface is its root port?

Answer: The root port is the port on which the BPDU with the lowest-cost value is received. The root port is placed in forwarding state on each bridge and switch.

4.Can the root bridge/switch ports be placed in blocking state?

Answer: The root bridge’s ports are always in forwarding state because they always have cost 0 to the root, which ensures that they are always the designated bridges on their respective LAN segments.

5.Describe the benefits of Spanning Tree Protocol as used by transparent bridges and switches.

Answer: Physically redundant paths in the network are allowed to exist and be used when other paths fail. Also, loops in the bridged network are avoided. Loops are particularly bad because bridging uses LAN headers, which do not provide a mechanism to mark a frame so that its lifetime can be limited; in other words, the frame can loop forever.

6.When a bridge or switch using Spanning Tree Protocol first initializes, what does it assert should be the tree’s root?

Answer: Each bridge/switch begins by sending BPDUs claiming itself as the root bridge.