18-4
Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 18 Configuring IGMP Snooping
Understanding IGMP Snooping
If another host (for example, Host 4) sends an unsolicited IGMP join message for the same group
(
Figure 18-2
), the CPU receives that message and adds the port number of Host 4 to the forwarding table
as shown in
Table 18-2
. Note that because the forwarding table directs IGMP messages to only the CPU,
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
Second Host Joining a Multicast Group
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:
•
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
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
Table 18-2
Updated IGMP Snooping Forwarding Table
Destination Address
Type of Packet
Ports
224.1.2.3
IGMP
1, 2, 5
Forwarding
table
CPU
Host 1
Host 2
Host 3
Host 4
Router A
PFC
VLAN
1
0
2
3
4
5
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