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C H A P T E R10
Virtual LANs and Trunking
It’s hard to be a networker today and not work with virtual LANs (VLANs) and VLAN trunking. Almost every campus LAN uses VLANs, and almost every campus LAN with more than one switch uses trunking. In short, you have to know these topics.
VLANs allow a switch to separate different physical ports into different groups so that traffic from devices in one group never gets forwarded to the other group. This allows engineers to build networks that meet their design requirements, without having to buy a different switch for each group. Also, multiple switches can be connected together, with traffic from multiple VLANs crossing the same Ethernet links, using a feature called trunking.
“Do I Know This Already?” Quiz
The purpose of the “Do I Know This Already?” quiz is to help you decide whether you really need to read the entire chapter. If you already intend to read the entire chapter, you do not necessarily need to answer these questions now.
The eight-question quiz, derived from the major sections in “Foundation Topics” portion of the chapter, helps you determine how to spend your limited study time.
Table 10-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?” quiz questions that correspond to those topics.
Table 10-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping
Foundations Topics Section
Virtual LAN Concepts
Trunking with ISL and 802.1q
Passing Traffic Between VLANs
Questions Covered in This Section
1, 7, 8
3, 4,
2, 5, 6
260 Chapter 10: Virtual LANs and Trunking
CAUTION The goal of self-assessment is to gauge your mastery of the topics in this chapter. If you do not know the answer to a question or are only partially sure of the answer, you should mark this question wrong for purposes of the self-assessment. Giving yourself credit for an answer that you correctly guess skews your self-assessment results and might provide you with a false sense of security.
1. In a LAN, which of the following terms best equates to the term VLAN?
a. Collision domain b. Broadcast domain c. Subnet domain
d. Single switch e. Trunk
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?
a. 0 b. 1 c. 2 d. 3
e. Can’t tell from the information provided
3. Which of the following fully encapsulates the original Ethernet frame in a trunking header?
a. VTP b. ISL
c. 802.1q
d. Both ISL and 802.1q e. None of the above
“Do I Know This Already?” Quiz 261
4. Which of the following allows a spanning tree instance per VLAN?
a. VTP b. ISL
c. 802.1q
d. Both ISL and 802.1q e. None of the above
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?
a. 0 b. 1 c. 2 d. 3
e. Can’t tell from the information provided
6. Which of the following terms refers to a function that can forward traffic between two different VLANs?
a. Layer 2 switching b. Layer 3 switching c. Layer 4 switching d. All of the above
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?
a. Collision domains b. IP subnets
c. Broadcast domains d. All of the above
e. None of the above
262 Chapter 10: Virtual LANs and Trunking
8. Which of the following are considered to be ways of configuring VLANs?
a. By statically assigning a switch port to a VLAN
b. By assigning a MAC address to a particular VLAN
c. By allowing DHCP to dynamically assign a PC to a particular VLAN d. By using the DVTP protocol
The answers to the “Do I Know This Already?” quiz are found in Appendix A, “Answers to the ‘Do I Know This Already?’ Quizzes and Q&A Sections.” The suggested choices for your next step are as follows:
6 or less overall score—Read the entire chapter. This includes the “Foundation Topics” and “Foundation Summary” sections and the Q&A section.
7 or 8 overall score—If you want more review on these topics, skip to the “Foundation Summary” section and then go to the Q&A section. Otherwise, move to the next chapter.
Virtual LAN Concepts 263
Foundation Topics
Virtual LAN Concepts
Before understanding VLANs, you must first have a very specific understanding of the definition of a LAN. Although you can think about LANs from many perspectives, one perspective in particular will help you with understanding VLANs:
A LAN includes all devices in the same broadcast domain.
As described in Chapter 9, “Cisco LAN Switching Basics,” a broadcast domain includes the set of all LAN connected devices that can send a broadcast frame, and all the other devices in the same LAN get a copy of the frame. So, you can think of a LAN and a broadcast domain as being basically the same thing.
Without VLANs, a switch treats all interfaces on the switch as being in the same broadcast domain—in others words, all connected devices are in the same LAN. With VLANs, a switch can put some interfaces into one broadcast domain and some into another. Essentially, the switch creates multiple broadcast domains. These individual broadcast domains created by the switch are called virtual LANs.
This chapter focuses on VLANs and the concepts and configuration required to implement VLANs on Cisco switches. This chapter covers VLAN concepts, including VLAN trunking. Also, you will read about what types of devices can be used to forward traffic between different VLANs.
VLAN Basics
A virtual LAN (VLAN) is a broadcast domain created by one or more switches. The switch creates a VLAN simply by putting some interfaces in one VLAN and some in another. So, instead of all ports on a switch forming a single broadcast domain, the switch separates them into many, based on configuration. It’s really that simple.
The first two figures in this chapter compare two networks. First, before VLANs existed, if a design specified two separate broadcast domains, two switches would be used—one for each broadcast domain, as shown in Figure 10-1.
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