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| 7.2 |  | Examining Multicast in a Multilayer Switched Network |
| 7.2.7 |  | IGMP version 2 |

Looking at the format of IGMPv2 messages, notice that the Type field has been merged with the IGMPv1 Version field and now occupies a full octet. The values assigned to the various message types have been chosen carefully to provide backward compatibility with IGMPv1. Another field added and not found in IGMPv1 is the Maximum Response Time.

IGMP Version 2 adds two messages not found in IGMPv1 to streamline the join and leave process. RFC 2236 added a Version 2 membership report and a leave group message. The complete list of messages now consists of the following:

- 0x11 – Membership query
- 0x12 – Version 1 membership report
- 0x16 – Version 2 membership report
- 0x17 – Leave group

The membership query and version 1 membership report carry over from IGMPv1. However, the membership query can now target specific multicast groups. In version 1, the query message is a general query with the group address set to 0.0.0.0. All active groups respond to the general query. Version 2 allows a multicast query router to target a specific multicast group and a multicast host, running version 2, will respond with a version 2 membership report. Other multicast groups ignore the group specific query if it is not directed to their group. The group specific membership query works only for version 2 systems.

If a version 2 host leaves a multicast group, it sends an unsolicited leave group message to inform the query router that it no longer desires to receive the multicast stream. The router maintains a table of all hosts in the multicast group on the segment. If other hosts still want to receive the multicast stream, the router continues to send the multicast frames onto the segment. If, however, the membership report arrives from the last host on the segment for the multicast group, the router terminates the multicast stream for that group.

In Figure , consider the multicast group shown. Hosts H2 and H3 are currently members of multicast group 224.1.1.1, although Host H2 wants to leave the group.

The sequence of events for Host H2 to leave the group is as follows:

- Host H2 multicasts an IGMPv2 leave group message to the All-Routes (224.0.0.2) multicast group to inform all routers on the subnet that it is leaving the group.
- Router A hears the leave group message from Host H2. However, because routers keep a list only of the group memberships that are active on a subnet, not individual hosts that are members, Router A sends a group-specific query to determine whether any hosts remain for Group 224.1.1.1. Since this is a group-specific query, only hosts that are members of the group will respond.
- Host H3 is still a member of Group 224.1.1.1 and responds to the query with an IGMPv2 membership report to inform the routers on the subnet that a member of this group is still present. Host H3 is now the last remaining member of the group.
- If Host H3 decides to leave, Steps 1 and 2 will be repeated. However, when Router A issues a group-specific query, no hosts will respond. As a result, Router A will time out the group and stop forwarding packets for the group onto the subnet.

If the router wants to confirm the need to send a stream, it can transmit a general or group-specific query onto the segment. If it does not receive any responses to a couple of query messages, the router assumes no more hosts want to receive the multicast stream.

Another feature added to IGMPv2 is the capability for the multicast router to specify the response timer range of the hosts. Remember that when a host receives a membership query, the host starts a random timer. The timer value is in the range of 0 to maximum response time, with the maximum response time specified in the router query message. Version 2 limits the upper range of the timer to a maximum of 25 seconds. The default in a router is 10 seconds.