Cisco Certified Internetwork Expert (CCIE) Practice Test

What is the purpose of IGMP leave messages in multicast group management?

1. To join new multicast groups

2. To signal a desire to stop receiving data for a multicast group

3. To maintain a list of active users in each multicast group

4. To assign IP addresses to new hosts in the group

The correct answer is: To signal a desire to stop receiving data for a multicast group

The purpose of IGMP (Internet Group Management Protocol) leave messages is to signal a desire to stop receiving data for a multicast group. When a host determines that it no longer wishes to receive multicast traffic from a specific group, it uses an IGMP leave message to notify the local router. This action helps the multicast routing infrastructure manage the distribution of multicast traffic efficiently, ensuring that resources are not wasted on hosts that are no longer interested in a particular stream of data. When a leave message is sent, the router can then decide whether it should prune the multicast group from its forwarding table. If there are no other members in that group, the router can stop forwarding packets to that multicast address, thus optimizing the network's bandwidth usage. The other options relate to functions that are not served by IGMP leave messages. For instance, joining new multicast groups is accomplished via IGMP join messages, while maintaining a list of active users is managed by the routers themselves as they process these join and leave messages. Assigning IP addresses to new hosts is also unrelated and typically handled through other protocols like DHCP.