IGMP and PIM Protocol Description
This section covers the packet-level details of IGMP and PIM. Because it is still being developed, MSDP is not discussed in detail here.
IGMP
IGMP messages are IP-encapsulated with a protocol number of 2, and are always sent with a TTL of 1 to ensure their confinement to the local LAN. IGMP versions 1, 2, and 3 are described here; however, remember that IGMP version 3 is still under development.
Five IGMP message types exist:
This message has the format shown in Figure 13-4, and is sent by the querying router:
11: IGMPv1/2/3 MEMBERSHIP QUERY
The group field may contain all zeros, which corresponds to a general query for membership information for all groups. This is the only message format supported by IGMPV1. ...
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