Chapter 18. IGMP

This chapter covers the following topics:

  • Data Link Layer Support for Multicast—Describes three methods to send multicasts to ports where the traffic is wanted and to hide multicasts from other hosts.

  • Understanding IGMP—Describes the features of IGMPv1, IGMPv2, and IGMPv3 and how to determine which IGMP version is running on a router.

  • Configuring IGMP—Describes enabling IGMP groups and configuring IGMP snooping.

Because not everyone on a network wants to receive the traffic from a multicast source, routers must have some means of forwarding traffic to only the destinations that want to receive it. This chapter covers IP multicast and some of the protocols used to forward multicast packets. You will learn the basic features and operation of IGMP and how to configure IGMP groups and IGMP snooping.

The purpose of the "Do I Know This Already?" quiz is to help you decide which parts of this chapter to use. If you already intend to read the entire chapter, you do not necessarily need to answer these questions.

The 11-question quiz, derived from the major sections in the "Foundation Topics" portion of the chapter, helps you determine how to spend your limited study time.

Table 18-1 outlines the major topics discussed in this chapter and the corresponding quiz questions.

Table 18-1. "Do I Know This Already?" Foundation Topics Section-to-Question Mapping
Foundation Topics SectionQuestions Covered in This SectionScore
Data Link Layer Support for Multicast1–4 
Understanding IGMP5–8 
Configuring IGMP9–11 
Total Score  


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 the question wrong for purposes of the self-assessment. Giving yourself credit for an answer you correctly guessed skews your results and might provide you with a false sense of security.

1.Layer 2 switches build a MAC table by observing which of the following?
  1. Source address and port

  2. Destination address and port

  3. Source VLAN

  4. Broadcast address and VLAN

2.IGMP stands for which of the following?
  1. Internal Group Multicast Protocol

  2. IP Group Management Protocol

  3. IP Group Maintenance Protocol

  4. Internet Group Membership Protocol

3.Which of the following is not a method of associating multicasts with ports?
  1. IGMP snooping

  2. CGMP

  3. CGMP snooping

  4. Static MAC Entries

4.Which of the following are differences between IGMP Snooping and CGMP?
  1. CGMP learns subscribing MAC addresses by passive listening.

  2. CGMP learns subscribing MAC addresses from the router.

  3. IGMP snooping is Cisco proprietary.

  4. IGMP snooping requires layer 3 processing.

5.What is the primary issue with IGMP version 1?
  1. Join and leave vacancy

  2. Join and leave latency

  3. It is proprietary

  4. It requires a BSR

6.Which of the following are advantages of IGMP v2 over IGMPv1?
  1. Group-specific queries

  2. Group Joins

  3. Group Leaves

  4. Source Filtering

7.IGMPv1 does not have a mechanism to allow a host to leave a group when it is no longer interested in the content of that group. Which of the following is an advantage of IGMPv3 over version 2?
  1. Group-specific queries

  2. Joins

  3. Leaves

  4. Source Filtering

8.What command displays the current IGMP version used on a port?
  1. show ip igmp interface

  2. show igmp port

  3. show ip igmp-snooping

  4. show multicast router

9.What command is used to manually join a router to a group?
  1. ip igmp interface

  2. ip multicast-group

  3. ip igmp join-group

  4. igmp join-group

10.How do you turn on IGMP Snooping?
  1. igmp-snooping

  2. ip igmp snooping

  3. igmp interface

  4. no ip cgmp

11.Which of the following commands displays the multicast groups used?
  1. ip igmp snooping

  2. show multicast group

  3. show multicast router

  4. show igmp interface

You can find the answers to the "Do I Know This Already?" quiz in Appendix A, "Answers to Chapter 'Do I Know This Already?' Quizzes and Q&A Sections." The suggested choices for your next step are as follows:

  • 8 or less overall score—Read the entire chapter. This includes the "Foundation Topics," "Foundation Summary," and "Q&A" sections.

  • 9 or 10 overall score—Begin with the "Foundation Summary" section and then go to the "Q&A" section at the end of the chapter. If you have trouble with these exercises, read the appropriate sections in "Foundation Topics."

  • 11 overall score—If you want more review on these topics, skip to the "Foundation Summary" section and then go to the "Q&A" section at the end of the chapter. Otherwise, move to the next chapter.