Leaving A Multicast Group - Cisco 3845 - Security Bundle Router Software Manual

Understanding IGMP Snooping
If another host (for example, Host 4) sends an unsolicited IGMP join message for the same group
(Figure
as shown in
the message is not flooded to other ports on the switch. Any known multicast traffic is forwarded to the
group and not to the CPU.
Figure 18-2
Table 18-2
Destination Address
224.1.2.3
Multicast-capable router ports are added to the forwarding table for every Layer 2 multicast entry. The
switch learns of such ports through one of these methods:
•
•

Leaving a Multicast Group

The router sends periodic multicast general queries, and the switch forwards these queries through all
ports in the VLAN. Interested hosts respond to the queries. If at least one host in the VLAN wishes to
receive multicast traffic, the router continues forwarding the multicast traffic to the VLAN. The switch
forwards multicast group traffic only to those hosts listed in the forwarding table for that IP multicast
group maintained by IGMP snooping.
When hosts want to leave a multicast group, they can silently leave, or they can send a leave message.
When the switch receives a leave message from a host, it sends a group-specific query to learn if any
other devices connected to that interface are interested in traffic for the specific multicast group. The
Cisco ME 3800X and 3600X Switch Software Configuration Guide
18-4
18-2), the CPU receives that message and adds the port number of Host 4 to the forwarding table
Table 18-2. Note that because the forwarding table directs IGMP messages to only the CPU,
Second Host Joining a Multicast Group
PFC
CPU
0
Forwarding
table
2
Host 1 Host 2
Updated IGMP Snooping Forwarding Table
Snooping on IGMP queries and Protocol Independent Multicast (PIM) packets
Statically connecting to a multicast router port with the ip igmp snooping mrouter global
configuration command
Router A
1
VLAN
3 4 5
Host 3 Host 4
Type of Packet Ports
IGMP 1, 2, 5
Chapter 18 Configuring IGMP Snooping
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