Do I Know This Already Quiz

1. l.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 would be 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.

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 allows a spanning tree instance per VLAN? Answer: D

5. Imagine a Layer 2 switch with three configured VLANs, using an external router for inter-VLAN traffic. What is the least number of router Fast Ethernet interfaces required to forward traffic between VLANs?

Answer: B. You can use one Fast Ethernet interface and use trunking between the router and the switch. A router is required to forward traffic between the VLANs.

Q&A

6. Which of the following terms refers to a function that can forward traffic between two different VLANs?

Answer: B and C. Layer 2 switching forwards frames only inside a single VLAN. Layer 3 switching and Layer 4 switching forward traffic between VLANs, either based on the Layer 3 destination address (Layer 3 switching) or the Layer 4 port numbers (Layer 4 switching).

7. Imagine a small campus network with three VLANs spread across two switches. Which of the following would you expect to also have a quantity of 3?

Answer: B and C. By definition, a VLAN is a set of devices in the same broadcast domain. An IP subnet on a LAN is typically comprised of devices in the same VLAN.

8. Which of the following are considered to be ways of configuring VLANs? Answer: A and B.

1. Define the term collision domain.

Answer: A collision domain is a set of Ethernet devices for which concurrent transmission of a frame by any two of them will result in a collision. Bridges, switches, and routers separate LAN segments into different collision domains. Repeaters and shared hubs do not separate segments into different collision domains.

2. Define the term 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.

3. Define the term VLAN.

Answer: Virtual LAN (VLAN) refers to 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 being routed by a router.

4. If two Cisco LAN switches are connected using Fast Ethernet, what VLAN trunking protocols could be used? If only one VLAN spanned both switches, is a VLAN trunking protocol needed?

Answer: ISL and 802.1q are the trunking protocols used by Cisco 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.

5. 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.

6. What is the acronym and complete name of Cisco's proprietary trunking protocol over Ethernet?

Answer: Inter-Switch Link (ISL).

7. Consider the phrase "A VLAN is a broadcast domain is an IP subnet." Do you agree or disagree? State your reasons.

Answer: From one perspective, the statement is false because an IP subnet is a Layer 3 protocol concept, and a broadcast domain and VLAN are Layer 2 concepts. However, the devices in one broadcast domain comprise the exact same set of devices that would be in the same VLAN and in the same IP subnet.

8. What fields are added or changed in an Ethernet header when using 802.1q? Where is the VLAN ID in those fields?

Answer: A new 4-byte 802.1q header, which 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.

9. Compare and contrast the use of a Layer 3 switch versus an external router connected to a Layer 2 switch using a trunk for forwarding between VLANs.

Answer: Functionally, the end result of each process is identical. However, L3 switches optimize the internal processing of the switch, using a very fast forwarding path, typically using ASICs. The end result is that L3 switches forward traffic between VLANs at much higher speeds than do externally attached routers.

10. Compare and contrast a Layer 3 switch with a multilayer switch. Describe in what cases the terms could be used synonymously.

Answer: A Layer 3 switch forwards packets based on their destination IP address, much like a router, but with the forwarding logic performed quickly inside hardware in the switch. A multilayer switch performs switching at multiple layers. Many Layer 3 switches also perform Layer 2 switching for frames destined to another device in the same VLAN, and Layer 3 switching for packets destined to another subnet/VLAN. The terms are often used synonymously because many people assume that a LAN switch can always perform L2 switching, with L3 switching being an additional function.

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