Configuring L2 Multicast Features 705
What Is IGMP Snooping?
IGMP Snooping is a layer 2 feature that allows the switch to dynamically add
or remove ports from IP multicast groups by listening to IGMP join and leave
requests. By "snooping" the IGMP packets transmitted between hosts and
routers, the IGMP Snooping feature enables the switch to forward IP
multicast traffic more intelligently and help conserve bandwidth.
Based on the IGMP query and report messages, the switch forwards traffic
only to the ports that request the multicast traffic. This prevents the switch
from broadcasting the traffic to all ports and possibly affecting network
performance. The switch uses the information in the IGMP packets as they
are being forwarded throughout the network to determine which segments
should receive packets directed to the group address.
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 IGMP snooping
functions on the VLAN.
When the IGMP snooping querier is enabled, the IGMP snooping querier
sends out periodic IGMP queries that trigger IGMP report messages from the
switch that wants to receive IP multicast traffic. The IGMP snooping feature
listens to these IGMP reports to establish appropriate forwarding.
What Is MLD Snooping?
In IPv4, Layer 2 switches can use IGMP snooping to limit the flooding of
multicast traffic by dynamically configuring L2 interfaces so that multicast
traffic is forwarded to only those interfaces associated with an IP multicast
address. In IPv6, MLD snooping performs a similar function. With MLD
snooping, IPv6 multicast data is selectively forwarded to a list of ports that
want to receive the data instead of being flooded to all ports in a VLAN. This
list is constructed by snooping IPv6 multicast control packets.
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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