Chapter 11 Configuring Igmp Snooping And Mvr; Joining A Multicast Group - Cisco WS-C3550-12G Software Configuration Manual

Understanding IGMP Snooping
the switch adds the host port number to the forwarding table entry; when it receives an IGMP Leave
Group message from a host, it removes the host port from the table entry. It also periodically deletes
entries if it does not receive IGMP membership reports from the multicast clients.
Note For more information on IP multicast and IGMP, refer to RFC 1112 and RFC 2236.
The multicast router (which could be a Catalyst 3550 switch with the enhanced multilayer software
image) sends out periodic general queries to all VLANs. All hosts interested in this multicast traffic send
join requests and are added to the forwarding table entry. The switch forwards only one join request per
IP multicast group to the multicast router. It creates one entry per VLAN in the Layer 2 forwarding table
for each MAC group from which it receives an IGMP join request.
Layer 2 multicast groups learned through IGMP snooping are dynamic. However, you can statically
configure MAC multicast groups by using the ip igmp snooping vlan static global configuration
command. If you specify group membership for a multicast group address statically, your setting
supersedes any automatic manipulation by IGMP snooping. Multicast group membership lists can
consist of both user-defined and IGMP snooping-learned settings.
Note If a spanning-tree VLAN topology change occurs, the IGMP snooping-learned multicast groups on
the VLAN are purged. Enabling spanning-tree Port Fast on direct-to-desktop ports stops STP
topology change notifications from being generated.

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
Catalyst 3550 Multilayer Switch Software Configuration Guide
11-2
Chapter 11 Configuring IGMP Snooping and MVR
Figure 11-1.
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