Cisco WS-X6066-SLB-APC - Content Switching Module Software Manual page 683

Chapter 40 Configuring Multicast Services
Joining a Multicast Group
When a host wants to join an IP multicast group, it sends an IGMP join (also known as a join message)
specifying the IP multicast group 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 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. Since the switch already has a group entry for 01-00-5e-01-02-03 in this VLAN, it just adds the
second host port to the entry. Because this is not the first host joining the group, the switch suppresses
the report (does not send it to the router).
Constraining Multicast Traffic
When a host sends multicast traffic to a group, the switch hardware does not recognize the stream as IGMP
control packets and therefore the packets are not redirected to the switch CPU. Instead the multicast traffic
hits the MAC group entry and the switch constrains the traffic to only those ports that have been added to that
group entry.
The router sends IGMP general queries every 60 seconds by default. The switch floods these queries on
all ports in the VLAN, and 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. Subsequent reports for the same VLAN and group are
suppressed (not sent to the router).
Note If there are CGMP switches in the network, join and leave suppression does not occur. In a network
that has both IGMP and CGMP switches, all join and leave messages are forwarded to the multicast
routers so that CGMP join and leave messages can be generated by the router.
Leaving a Multicast Group
The designated multicast router for a segment continues forwarding the multicast traffic to that VLAN as long
as at least one host in the VLAN wishes to receive multicast traffic. When hosts want to leave a multicast
group, they can either ignore the periodic general queries sent by the multicast router (IGMP v1 host
behavior), or they can send an IGMP leave (IGMP v2 host behavior). When the switch receives a leave
message, it sends out a MAC-based general query on the port on which it received the leave message to
determine if any devices connected to this port are interested in traffic for the specific multicast group. If this
port is the last port in the VLAN, the switch sends a MAC-based general query to all ports in the VLAN.
MAC-based general queries are addressed to the Layer 2 Group Destination Address (GDA) MAC
address for which the IGMP leave message was received. At Layer 3, the MAC-based general queries
are addressed to 244.0.0.1 (all hosts), and in the IGMP header, the group address field is set to 0.0.0.0.
If no IGMP join is received for any of the IP multicast groups that map to the MAC multicast group address,
the port is removed from the multicast forwarding entry. If the port is not the last non-multicast-router
port in the entry, the switch suppresses the IGMP leave (does not send it to the router). If the port is the last
non-multicast-router port in the entry, the IGMP leave is forwarded to the multicast router ports and the
MAC group forwarding entry is removed.
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Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
Understanding How Multicasting Works
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