Answers to Chapter 13 Review Questions

Answers to Chapter 13 Review Questions

1:

IGMP version 2 includes an explicit leave message for hosts to transmit whenever they no longer want to receive a multicast stream. Why, then, does version 2 include the query message?

A:

The query message remains in version 2 for three reasons. One reason is for backwards compatibility with version 1. Another reason is to enable the router to be absolutely sure that no hosts exist that intend to receive the stream. It is possible that a leave or join message can be lost from a collision or other physical layer event causing the router to erroneously believe that it should terminate the stream. The query message, then, is an insurance policy. A third reason for retaining the query message is to support the query router selection process. Only one router per segment can be a query router. In version 2, the router with the lowest IP address becomes the query router.

2:

Why doesn't a Catalyst normally learn multicast addresses?

A:

The Catalyst, a bridge, learns source addresses. Multicast addresses never appear in the source address field of a frame.

3:

What Layer 2, Layer 3, and IGMP information does a multicast device transmit for a membership report?

A:

A membership includes the following:

• The Layer 2 header uses the sources unicast address in the source field and the calculated multicast MAC address in the destination field.

• The Layer 3 header uses the source's IP address and the multicast group address for the destination.

The IGMP membership report uses the group multicast address.

4:

Assume that you have a switched network with devices running IGMP version 1 and the switches/routers have CGMP enabled. One of the multicast devices surfs the Web looking for a particular multicast stream. The user first connects to group 1 and finds it isn't the group that he wants. So he tries group 2, and then group 3, until he finally finds what he wants in group 4. Meanwhile, another user belongs to groups 1, 2, and 3. What happens to this user's link?

A:

The user's link continues to carry traffic from all four multicast groups until there are no members in the broadcast domain for those groups. CGMP and IGMP version 1 cannot remove a user from a multicast stream until there are no more active members of the group. This stems from the implicit leave function of IGMP version 1. This can create a bandwidth problem for the user because he might have four multicast streams hitting his interface.