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| 7.2 |  | Examining Multicast in a Multilayer Switched Network |
| 7.2.6 |  | IGMP version 1 |

IGMPv1 messages are transported within IP datagrams to transmit information about multicast groups. The datagram consists of a 20-byte IP header and an 8-byte IGMP message.

The first field of the message indicates what version of IGMP generated the message. For Version 1, this value must be 1. The next field specifies the message type. Version 1 defines two messages: a host membership query and a host membership report. The Checksum field carries the checksum computed by the source; the source computes the checksum for the entire IGMP message. The receiving device examines the checksum value to determine if the message was corrupted during transmission. If the checksum value doesn’t match, the receiver discards the message. The final field indicates the multicast destination group address targeted by the message.

When a host on the segment wants to receive multicast traffic, it issues an unsolicited host membership report targeting the intended multicast group.

The destination MAC address targets the multicast group it intends to join. If this were the only information in the frame to identify the group, any of 32 groups might be desired because of the address ambiguity discussed earlier. However, the Layer 3 address is included in both the IP header and in the IGMP header. The Layer 3 multicast group desired by the host is 239.255.160.171. This translates to a MAC address of 01-00-5E-7F-A0-AB. All multicast-capable devices on the shared media receive the membership report. In this situation, however, only the router is interested in the frame. The frame tells the router, “I want to receive any messages for this multicast group.” The router now knows that it needs to forward a copy of any frames with this multicast address to the segment where the host that issued the report lives.

A device issues an IGMP membership report under two conditions:

- Whenever the device first intends to receive a multicast stream – When enabling the multicast application, the device configures the NIC, and the built-in IGMP processes send an unsolicited membership report to the router requesting copies of the multicast frames.
- In response to a membership query from the router – This is a solicited membership report that helps the router to confirm that hosts on the segment still want to receive multicast traffic for a particular multicast group.

Routers periodically issue host membership queries. The query period may be configured from 0 seconds to a maximum of 65,535 seconds. The default is 60 seconds.

Only one router on each segment issues the membership query message. If the segment uses IGMP Version 1, the designated router for the multicast routing protocol issues the query. If the segment uses IGMP Version 2, the multicast router with the lowest IP address on the segment issues the queries.

The host membership query message targets the all multicast hosts address, 224.0.0.1, in the Layer 3 address field. The Layer 2 address is 01-00-5E-00-00-01. The IGMP header uses a group address of 0.0.0.0. This translates to a query for all multicast hosts on all groups on the segment.

A host for each active multicast group must respond to this message. When multicast hosts receive the host membership query, and if the host wants to continue to receive multicast frames for a multicast group, the multicast host starts an internal random timer with an upper range of 10 seconds. The host waits for the timer to expire and then it sends a membership report for each multicast group that it wants to continue to receive.

If another station sends a membership report before the local timer expires, the host cancels the timer and suppresses the report. This behavior prevents a segment and a router from experiencing host membership report floods. Only one station from each multicast group needs to respond for each group on the segment.

If the router does not receive a membership report for a particular multicast group for three query intervals, the router assumes no hosts remain that are interested in that group multicast stream. The router stops forwarding the multicast packets for this group and informs upstream routers to stop sending frames.

This process defines an implicit leave from the multicast group. No special leave mechanism was defined in IGMPv1. Instead, IGMPv1 hosts leave the group passively without any notification to the router. A router learns that such a host is not interested in participating in a multicast group if the host does not respond to repeated queries from the router relating to that group.