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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
IGMP Versions 1 and 2
Users that want to receive multicast packets need to be able to join and leave multicast groups. This is
accomplished using IGMP.
Figure 5-18. IGMP Message Format
The IGMP Type codes are shown below:
Type Meaning
0x11
Membership Query (if Group Address is 0.0.0.0)
0x1
1
Specific Group Membership Query (if Group Address
is Present)
0x1
6
Membership Report (version 2)
0x1
7
Leave a Group (version 2)
0x1
2
Membership Report (version 1)
Table 5-11. IGMP Type Codes
Multicast routers use IGMP to manage multicast group memberships:
• An IGMP “report” is sent by a user’s computer to join a group
• IGMP version 1 does not have an explicit ‘leave’ message. Group members have an expiration
timer, and if this timer expires before a query response is returned, the member is dropped from
the group.
• IGMP version 2 introduces an explicit “leave” report. When a user wants to leave a group, this
report is sent to the multicast router (for IGMP version 2).
• Multicast routers send IGMP queries (to the all-hosts group address: 224.0.0.1) periodically to
see whether any group members exist on their subnetworks. If there is no response from a
particular group, the router assumes that there are no group members on the network, and
multicast packets are not forwarded.
The TTL field of query messages is set to 1 so that the queries do not get forwarded to other
subnetworks.
IGMP version 2 introduces a few extensions to IGMP version 1 such as, the election of a single
multicast querier for each network, explicit ‘leave’ reports, and queries that are specific to a particular
multicast group.
The router with the lowest IP address is elected as the querier. The explicit group leave message is
added to decrease latency, and routers can ask for membership reports from a particular multicast
group ID.
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