Igmp Versions; Joining A Multicast Group - Cisco Catalyst 3750-X Software Configuration Manual

Chapter 26 Configuring IGMP Snooping and MVR

IGMP Versions

The switch supports IGMP Version 1, IGMP Version 2, and IGMP Version 3. These versions are
interoperable on the switch. For example, if IGMP snooping is enabled on an IGMPv2 switch and the
switch receives an IGMPv3 report from a host, the switch can forward the IGMPv3 report to the
multicast router.
The switch supports IGMPv3 snooping based only on the destination multicast MAC address. It does not
Note
support snooping based on the source MAC address or on proxy reports.
An IGMPv3 switch supports Basic IGMPv3 Snooping Support (BISS), which includes support for the
snooping features on IGMPv1 and IGMPv2 switches and for IGMPv3 membership report messages.
BISS constrains the flooding of multicast traffic when your network includes IGMPv3 hosts. It
constrains traffic to approximately the same set of ports as the IGMP snooping feature on IGMPv2 or
IGMPv1 hosts.
IGMPv3 join and leave messages are not supported on switches running IGMP filtering or MVR.
Note
An IGMPv3 switch can receive messages from and forward messages to a device running the Source
Specific Multicast (SSM) feature. For more information about source-specific multicast with IGMPv3
and IGMP, see this URL:
http://www.cisco.com/en/US/products/sw/iosswrel/ps1834/products_feature_guide09186a008008048a.
html

Joining a Multicast Group

When a host connected to the switch wants to join an IP multicast group and it is an IGMP Version 2
client, it sends an unsolicited IGMP join message, specifying the IP multicast group to join.
Alternatively, when the switch receives a general query from the router, it forwards the query to all ports
in the VLAN. IGMP V ersion 1 or Version 2 hosts wanting to join the multicast group respond by sending
a join message to the switch. The switch CPU creates a multicast forwarding-table entry for the group if
it is not already present. The CPU also adds the interface where the join message was received to the
forwarding-table entry. The host associated with that interface receives multicast traffic for that
multicast group. See
Figure 26-1
Router A sends a general query to the switch, which forwards the query to ports 2 through 5, which are
all members of the same VLAN. Host 1 wants to join multicast group 224.1.2.3 and multicasts an IGMP
membership report (IGMP join message) to the group. The switch CPU uses the information in the IGMP
report to set up a forwarding-table entry, as shown in
connected to Host 1and the router.
Table 26-1
Destination Address
224.1.2.3
OL-21521-01
Figure 26-1.
Initial IGMP Join Message
IGMP Snooping Forwarding Table
Type of Packet
IGMP
Catalyst 3750-X and 3560-X Switch Software Configuration Guide
Understanding IGMP Snooping
Table 26-1, that includes the port numbers
Ports
1, 2
26-3