Operation Manual – Multicast
H3C S3600 Series Ethernet Switches-Release 1510 Chapter 6 IGMP Configuration
6-2
querier election mechanism is required to determine which router will act as the IGMP
querier on the subnet.
In IGMPv1, the designated router (DR) elected by the Layer 3 multicast routing protocol
(such as PIM) serves as the IGMP querier.
Figure 6-1 Work mechanism of IGMPv1
Assume that Host B and Host C are expected to receive multicasts address to multicast
group G1, while Host A is expected to receive multicasts address to G2, as shown in
Figure 6-1. The hosts join the multicast group in a process described below:
1) The IGMP querier (DR in the figure) periodically sends IGMP queries (with the
destination address of 224.0.0.1) to all hosts and routers on the same subnet.
2) Upon receiving a query message, either Host B or Host C (the delay timer of
whichever expires first) that is of concern to G1 sends an IGMP report first, with
the destination address being the group address of G1, to announce that it will join
G1. Assume it is Host B that sends the report message.
3) Because Host C is also interested in G1, it also receives the report that Host B
sends to G1. Upon receiving the report, Host C will suppress itself from sending
the same G1-specific message, because the IGMP routers already know that a
host on the subnet is interested in G1. This IGMP report suppression mechanism
helps reduce traffic over the local subnet.
4) Meanwhile, because Host A is interested in G2, it sends a report (with the group
address of G2 as the destination address) to announce that it will join G2.
5) Through the query/response process, the IGMP routers learn about the receivers
corresponding to G1 and G2 on the local subnet, and generate (*, G1) and (*, G2)
multicast forwarding entries as the basis for forwarding the multicast information,
where * represents any multicast source.
6) When the multicast data addressed to G1 or G2 reaches an IGMP router, because
the (*, G1) and (*, G2) multicast forwarding entries exist on the IGMP router, the
To Next Page
Loading...
Need help?
General