698 Configuring L2 Multicast Features
Multicast routers send an IGMP query every 60 seconds. This query is
intercepted by the switch and forwarded to all ports in the VLAN. All hosts
that are members of the group answer that query. The switch intercepts the
replies and forwards only one report per group from all of the received
responses.
In summary:
• IGMP snooping controls the flooding/forwarding behavior for multicast
groups. Multicast data is flooded in the VLAN until a multicast router port
is identified.
• IGMP snooping is enabled by default
• Multicast filtering is enabled by default
• IGMP snooping forwards multicast sources to multicast routers by default
• Reserved multicast IP addresses (224.0.0.x) are always flooded to all ports
in the VLAN
• Unregistered multicast traffic may be flooded in the VLAN by a user
configuration option.
IGMP Snooping Querier
When PIM and IGMP are enabled in a network with IP multicast routing, the
IP multicast router acts as the IGMP querier. However, if the IP-multicast
traffic in a VLAN needs to be Layer 2 switched only, an IP-multicast router is
not required. The IGMP snooping querier can perform the role of generating
IGMP queries that would normally be performed by the multicast router.
When IGMP snooping querier is enabled, the querier switch sends out
periodic IGMP queries that trigger IGMP report messages from the hosts that
want to receive IP multicast traffic. The IGMP snooping feature listens to
NOTE: It is strongly recommended that users enable MLD snooping if IGMP
snooping is enabled and vice-versa. This is because both IGMP snooping and
MLD snooping utilize the same forwarding table and not enabling both may
cause unwanted pruning of protocol packets utilized by other protocols, e.g.
OSPFv3.
NOTE: Without an IP-multicast router on a VLAN, you must configure another
switch as the IGMP querier so that it can send queries.
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