20-3
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
78-11380-12
Chapter 20 Configuring IGMP Snooping and MVR
Understanding IGMP Snooping
Note The switches support IGMPv3 snooping based only on the destination multicast MAC address. They do
not 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.
Note IGMPv3 join and leave messages are not supported on switches running IGMP filtering or MVR.
An IGMPv3 switch can receive messages from and forward messages to a device running the Source
Specific Multicast (SSM) feature. For more information, see the “Configuring IP Multicast Layer 3
Switching” chapter in the Catalyst 4500 Series Switch Cisco IOS Software Configuration Guide, Cisco
IOS Release 12.1(12c)EW at this URL:
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat4000/12_1_12/config/mcastmls.htm
Joining a Multicast Group
When a host connected to the switch wants to join an IP multicast group, 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. 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 20-1.
Figure 20-1 Initial IGMP Join Message
Forwarding
table
CPU
Host 1 Host 2 Host 3 Host 4
Router A
IGMP report 224.1.2.3
VLAN
PFC
1
0
2345
45750
To Next Page
Loading...
