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Configuring L2 Multicast Features 805
When a packet with a broadcast or multicast destination MAC address is
received, the switch will flood a copy into each of the remaining network
segments in accordance with the IEEE MAC Bridge standard. Eventually, the
packet is made accessible to all nodes connected to the network.
This approach works well for broadcast packets that are intended to be seen or
processed by all connected nodes. In the case of multicast packets, however,
this approach could lead to less efficient use of network bandwidth,
particularly when the packet is intended for only a small number of nodes.
Packets will be flooded into network segments where no node has any interest
in receiving the packet.
What Is IGMP Snooping?
IGMP snooping allows the switch to snoop on IGMP exchanges between
hosts and multicast routers and perform multicast forwarding within a VLAN.
The IGMP snooping feature complies with RFC 4541. When a switch “sees”
an IGMP report from a host for a given multicast address, the switch adds the
host's interface/VLAN to the L2 multicast group forwarding table and floods
the report to all ports in the VLAN. When the switch sees a leave message for
the group, it removes the host interface/VLAN from the L2 multicast group
forwarding table.
IGMP snooping learns about multicast routers by listening for the following
messages:
• An IGMP query packet.
• PIMv1 (IGMP type 0x14) packets with destination IP address 224.0.0.13.
• DVMRP (IGMP type 0x13) packets with destination IP address 224.0.0.4.
• PIMv2 (IP protocol type 0x67) packets with destination IP address
224.0.0.13.
Group addresses that fall into the range 224.0.0.x are never pruned by IGMP
snooping—they are always flooded to all ports in the VLAN. Note that this
flooding is based on the IP address, not the corresponding 01-00-5e-00-00-xx
MAC address.
When a multicast router is discovered, its interface is added to the interface
distribution list for all multicast groups in the VLAN. If a switch is connected
to a multicast source and no client, the switch filters the traffic from that
group to all interfaces in the VLAN. If the switch sees an IGMP join from a
host in the same VLAN, then it forwards the traffic to the host. Likewise, if