Igmp Snooping Overview; Joining A Multicast Group - Cisco 7609 Configuration Manual

Understanding How IGMP Snooping Works

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 multicast traffic at Layer 2 by
configuring Layer 2 LAN ports dynamically to forward 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."
With Release 12.1(8a)E and later releases, you can configure the IGMP snooping querier on the router
to support IGMP snooping in subnets that do not have any multicast router interfaces because the
multicast traffic does not need to be routed. 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 switch forwards through all ports in the VLAN and to which hosts
respond. IGMP snooping monitors the Layer 3 IGMP traffic.
Note If a multicast group has only sources and no receivers in a VLAN, IGMP snooping constrains the
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 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—12.1E
21-2
"Enabling the IGMP Snooping Querier" section on page
Chapter 21
Chapter 18, "Configuring IP
21-7.
Figure 21-1.
Configuring IGMP Snooping
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