Users Guide

Table Of Contents
Layer-2 Multicast Features 929
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
these IGMP reports to identify multicast router ports. If there is another
querier in the network and the local querier is in election mode, then the
querier with the lower IP address is elected and the other querier stops
querying. If the local querier is not in election mode and another querier is
detected, the local querier stops querying.
What Is MLD Snooping?
In IPv4, Layer-2 switches use IGMP snooping to limit the flooding of
multicast traffic by dynamically configuring the multicast forwarding
database so that multicast data traffic is forwarded to only those ports
NOTE: It is strongly recommended that operators 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. Not enabling both may cause
unwanted pruning of protocol packets utilized by other protocols, e.g. OSPFv3.
NOTE: IGMP snooping (and IGMP querier) validates IGMP packets. As part of
the validation, IGMP checks for the router alert option. If other devices in the
network do not send IGMP packets with the router alert option, IGMP snooping
(and snooping querier) will discard the packet. Use the no ip igmp snooping
router-alert-check command to disable checking for the router alert option.
NOTE: Without an IP-multicast router on a VLAN, you must configure another
switch as the IGMP querier so that it can send queries.