Cisco WS-C6506 Software Manual page 1196

Understanding How Multicasting Works
Joining a Multicast Group
In IGMP version 2, when a host wants to join an IP multicast group, it either responds to a router query
or sends an IGMP join (also known as a join message) specifying the IP multicast group to which it wants
to join (for example, group 224.1.2.3). The switch hardware recognizes that the packet is an IGMP report
and redirects it to the switch CPU. The switch installs a new group entry for 01-00-5e-01-02-03 and adds
the host port and the router port to that entry. The switch then relays the join from the host to all multicast
router ports. The designated multicast router for the segment adds the outgoing interface (OIF) to the
outgoing interface list (OIL) for the group and begins forwarding the multicast traffic for 224.1.2.3 to
this segment.
When a second host in this VLAN wants to join group 244.1.2.3, it sends out an IGMP join for this
group. The switch hardware recognizes that this is an IGMP control packet and redirects it to the switch
CPU. Because the switch already has a group entry for 01-00-5e-01-02-03 in this VLAN, it only adds
the second host port to the entry. Because this is not the first host joining the group, the switch suppresses
the report (the switch does not send it to the router).
The IGMP version 3 reports are sent by the hosts to the 224.0.0.22 address. The multicast router keeps
a state record for each group on an interface, and the switch maintains a state record for each group on
a per-VLAN basis. The state records contain the multicast IP address, the group timer, the source timer,
and the filter mode as specified by the hosts. The hosts can specify one of the following filter modes:
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Note If IP MMLS is disabled, the IGMP compatibility mode changes to version 1 or version 2 as soon as
version 1 or version 2 messages are received for a group on a VLAN (where the version 3 state
previously existed for that particular group on that VLAN).
Constraining Multicast Traffic
When a host sends the multicast traffic to a group, the switch hardware does not recognize the stream as IGMP
control packets. The packets are not redirected to the switch CPU. Instead, the multicast traffic is forwarded
to the Media Access Control (MAC) group entry and the switch constrains the traffic to only those ports that
have been added to that group entry.
The router sends the IGMP general queries. The switch floods these queries on all ports in the VLAN,
and the hosts that are interested in a multicast group respond with an IGMP join for each group in which
they are interested.
The switch intercepts these IGMP joins, and only the first join per VLAN and per IP multicast group is
forwarded on the multicast router ports. The subsequent reports for the same VLAN and group are
suppressed (not sent to the router). If you enable the switch for IGMP version 3 snooping, all joins are
forwarded to the router ports.
If you have CGMP switches in your network, join and leave suppression does not occur. In a network
Note
that has both IGMP version 2 and CGMP switches, all join and leave messages are forwarded to the
multicast routers so that the CGMP join and leave messages can be generated by the router.
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
51-4
INCLUDE mode—In this mode, the host announces membership to a multicast group and provides
a list of source IP addresses (the INCLUDE list) from which it wants to receive the traffic.
EXCLUDE mode—In this mode, the host announces membership to a multicast group and provides
a list of source IP addresses (the EXCLUDE list) from which it does not want to receive the traffic.
This mode indicates that the host wants to receive the traffic only from those sources with IP
addresses that are not listed in the EXCLUDE list. To receive the traffic from all sources, a host
expresses EXCLUDE mode membership with an empty EXCLUDE list.
Chapter 51 Configuring Multicast Services
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