Igmp Snooping Overview; Joining A Multicast Group - Cisco 7604 Configuration Manual
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Understanding How IGMP Snooping Works
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IGMP Snooping Overview
You can configure the router to use IGMP snooping in subnets that receive IGMP queries from either
IGMP or the IGMP snooping querier. IGMP snooping constrains IPv4 multicast traffic at Layer 2 by
configuring Layer 2 LAN ports dynamically to forward IPv4 multicast traffic only to those ports that
want to receive it.
IGMP, which runs at Layer 3 on a multicast router, generates Layer 3 IGMP queries in subnets where
the multicast traffic needs to be routed. For information about IGMP, see
Multicast Layer 3 Switching."
You can configure the IGMP snooping querier on the router to support IGMP snooping in subnets that
do not have any multicast router interfaces. For more information about the IGMP snooping querier, see
the
IGMP (on a multicast router) or the IGMP snooping querier (on the supervisor engine) sends out periodic
general IGMP queries that the router forwards through all ports in the VLAN and to which hosts respond.
IGMP snooping monitors the Layer 3 IGMP traffic.
If a multicast group has only sources and no receivers in a VLAN, IGMP snooping constrains the
Note
multicast traffic to only the multicast router ports.
Joining a Multicast Group
Hosts join multicast groups either by sending an unsolicited IGMP join message or by sending an IGMP
join message in response to a general query from a multicast router (the router forwards general queries
from multicast routers to all ports in a VLAN).
In response to an IGMP join request, the router creates an entry in its Layer 2 forwarding table for the
VLAN on which the join request was received. When other hosts that are interested in this multicast
traffic send IGMP join requests, the router adds them to the existing Layer 2 forwarding table entry. The
router creates only one entry per VLAN in the Layer 2 forwarding table for each multicast group for
which it receives an IGMP join request.
IGMP snooping suppresses all but one of the host join messages per multicast group and forwards this
one join message to the multicast router.
The router forwards multicast traffic for the multicast group specified in the join message to the
interfaces where join messages were received (see
Layer 2 multicast groups learned through IGMP snooping are dynamic. However, you can statically
configure Layer 2 multicast groups using the mac-address-table static command. When you specify
group membership for a multicast group address statically, the static setting supersedes any IGMP
snooping learning. Multicast group membership lists can consist of both static and IGMP
snooping-learned settings.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
30-2
Leaving a Multicast Group, page 30-4
Understanding the IGMP Snooping Querier, page 30-5
Understanding IGMP Version 3 Support, page 30-5
"Enabling the IGMP Snooping Querier" section on page
Chapter 30
Configuring IGMP Snooping for IPv4 Multicast Traffic
Chapter 28, "Configuring IPv4
30-9.
Figure
30-1).
OL-4266-08
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