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7.Name the three reasons why a port is placed in forwarding state as a result of spanning tree.
Answer: First, all ports on the root bridge are placed in forwarding state. Second, one port on each bridge is considered its root port, which is placed in forwarding state. Finally, on each LAN segment, one bridge is considered the designated bridge on that LAN; that designated bridge’s interface on the LAN is placed in forwarding state.
8.Name the three interface states that Spanning Tree Protocol uses, other than forwarding. Which of these states is transitory?
Answer: Blocking, listening, and learning. Blocking is the only stable state; the other two are transitory between blocking and forwarding. Table 2-2 summarizes the states and their features.
9.What are the two reasons that a nonroot bridge/switch places a port in forwarding state?
Answer: If the port is the designated bridge on its LAN segment, the port is placed in forwarding state. Also, if the port is the root port, it is placed in forwarding state. Otherwise, the port is placed in blocking state.
10.Which two 2950 series EXEC commands list information about an interface’s spanningtree state?
Answer: The show spanning-tree command lists details of the current spanning tree for all VLANs, including port status. show spanning-tree interface x/y lists the details just for interface x/y.
Chapter 3
“Do I Know This Already?” Quiz
1.In a LAN, which of the following terms best equates to the term “VLAN”?
Answer: B. By definition, a VLAN includes all devices in the same LAN broadcast domain.
2.Imagine a switch with three configured VLANs. How many IP subnets are required, assuming that all hosts in all VLANs want to use TCP/IP?
Answer: D. The hosts in each VLAN must be in different subnets.
502Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Questions
3.Which of the following fully encapsulates the original Ethernet frame in a trunking header?
Answer: B. ISL fully encapsulates the original frame, whereas 802.1q simply adds an additional header inside the original Ethernet frame.
4.Which of the following adds the trunking header for all VLANs except one?
Answer: C. 802.1q treats one VLAN as the “native” VLAN. It does not add the trunking header for frames in the native VLAN.
5.Which of the following allows a spanning tree instance per VLAN?
Answer: D
6.Which of the following advertises VLAN information to neighboring switches?
Answer: A. The primary feature of VTP is to distribute VLAN configuration information.
7.Which of the following VTP modes allow VLANs to be created on a switch?
Answer: B, C
8.Imagine that you are told that switch 1 is configured with the auto parameter for trunking on its Ethernet connection to switch 2. You have to configure switch 2. Which of the following settings for trunking could allow trunking to work?
Answer: A, C. Auto means that a switch waits for the switch on the other end of the trunk to attempt trunking first. With auto set on both ends of the trunk, trunking would never work.
Q&A
1.Define broadcast domain.
Answer: A broadcast domain is a set of Ethernet devices for which a broadcast sent by any one of them should be received by all others in the group. Unlike routers, bridges and switches do not stop the flow of broadcasts. Two segments separated by a router would each be in a different broadcast domain. A switch can create multiple broadcast domains by creating multiple VLANs, but a router must be used to route packets between the VLANs.
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2.Define VLAN.
Answer: A virtual LAN is the process of treating one subset of a switch’s interfaces as one broadcast domain. Broadcasts from one VLAN are not forwarded to other VLANs; unicasts between VLANs must use a router. Advanced methods, such as Layer 3 switching, can be used to allow the LAN switch to forward traffic between VLANs without each individual frame’s being routed by a router. However, for the depth of CCNA, such detail is not needed.
3.If two Cisco LAN switches are connected using Fast Ethernet, what VLAN trunking protocols can be used? If only one VLAN spans both switches, is a VLAN trunking protocol needed?
Answer: ISL and 802.1q are the trunking protocols that Cisco uses over Fast Ethernet. If only one VLAN spans the two switches, a trunking protocol is not needed. Trunking or tagging protocols are used to tag a frame as being in a particular VLAN; if only one VLAN is used, tagging is unnecessary.
4.Define VTP.
Answer: VLAN Trunking Protocol transmits configuration information about VLANs between interconnected switches. VTP helps prevent misconfiguration, eases switch administration, and reduces broadcast overhead through the use of VTP pruning.
5.Name the three VTP modes. Which mode does not allow VLANs to be added or modified?
Answer: Server and client modes are used to actively participate in VTP; transparent mode is used to simply stay out of the way of servers and clients while not participating in VTP. Switches in client mode cannot change or add VLANs.
6.What Catalyst 2950 switch command configures ISL trunking on fastethernet port 0/12 so that as long as the switch port on the other end of the trunk is not disabled (off) or configured to not negotiate to become a trunk, the trunk is definitely placed in trunking mode?
Answer: The switchport mode dynamic desirable interface subcommand tells this switch to be in trunking mode as long as the switch on the other end of the trunk is configured for trunk, auto, or desirable. If the other switch has configured the trunk as an access port, the interface does not use trunking.
7.What type of VTP mode allows a switch to create VLANs and advertise them to other switches?
Answer: Only VTP servers can create and advertise VLANs with VTP.
504Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Questions
8.Must all members of the same VLAN be in the same collision domain, the same broadcast domain, or both?
Answer: By definition, members of the same VLAN are all part of the same broadcast domain. They might all be in the same collision domain, but only if all devices in the VLAN are connected to hubs.
9.What is Cisco’s proprietary trunking protocol over Ethernet?
Answer: Inter-Switch Link (ISL)
10.Explain the benefits provided by VTP pruning.
Answer: VTP pruning reduces network overhead by preventing broadcasts and unknown unicast frames in a VLAN from being sent to switches that have no interfaces in that VLAN.
11.Consider the phrase “A VLAN is a broadcast domain is an IP subnet.” Do you agree or disagree? Why?
Answer: From one perspective, this statement is false, because an IP subnet is a Layer 3 protocol concept, and broadcast domain and VLAN are Layer 2 concepts. However, the devices in one broadcast domain comprise the same set of devices that would be in the same VLAN and in the same IP subnet.
12.What fields are added or changed in an Ethernet header when you use 802.1q? Where is the VLAN ID in those fields?
Answer: A new 4-byte 802.1q header that includes the VLAN ID is added after the source MAC address field. The original FCS field in the Ethernet trailer is modified, because the value must be recalculated as a result of changing the header.
13.Explain how a switch in VTP transparent mode treats VTP messages received from a VTP server.
Answer: A switch in VTP transparent mode receives the VTP messages and forwards them as broadcasts. However, the switch ignores the contents of the messages, so it does not learn any VLAN information from the messages.
14.What command on a 2950 switch creates VLAN 5? What configuration mode is required?
In VLAN database configuration mode, the vlan 5 name newvlan5 command would create the new vlan, and give it a name.
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15.What command on a 2950 switch puts an interface into VLAN 5? What configuration mode is required?
Answer: In interface configuration mode for that interface, the command switchport access vlan 5 assigns the interface to VLAN 5.
16.Describe the basic differences in the processes used by VLAN configuration mode and the normally used configuration mode.
Answer: In VLAN configuration mode, the commands do not take immediate effect. You must exit configuration mode or use the apply command to cause the configuration to be accepted.
17.Give the correct syntax for the commands that put an interface into the various trunking modes, and identify which commands work when the switch on the other side of the link uses the auto option.
Answer:
switchport mode dynamic desirable switchport mode dynamic auto switchport mode trunk
switchport mode access
The first and third commands work with auto set on the other side of the link.
18.What 2950 show commands list trunk status, both configured and operational?
Answer:
show interfaces fastethernet 0/x switchport show interfaces fastethernet 0/x trunk