IP Multicast Filtering D-1
DIP Multicast Filtering
The Internet Group Management Protocol (IGMP) runs between hosts and their immediately
neighboring multicast router/switch. The protocol’s mechanisms allow a host to inform its local
router that it wants to receive transmissions addressed to a specific multicast group.
A router, or multicast-enabled switch, can periodically ask their hosts if they want to receive
multicast traffic. If there is more than one router/switch on the LAN performing IP multicasting,
one of these devices is elected “querier” and assumes the responsibility of querying the LAN for
group members.
Based on the group membership information learned from IGMP, a router/switch can determine
which (if any) multicast traffic needs to be forwarded to each of its ports. At Layer-3, multicast
routers use this information, along with a multicast routing protocol, to support IP multicasting
across the Internet.
IGMP provides the final step in an IP multicast packet delivery service since it is only concerned
with forwarding multicast traffic from the local router/switch to group members on a directly
attached subnetwork or LAN segment.
This switch supports IP Multicast Filtering by
•passively snooping on the IGMP Query and IGMP Report packets transferred between IP
Multicast Routers and IP Multicast host groups to learn IP Multicast group members, and
•actively sending IGMP Query messages to solicit IP Multicast group members.
The purpose of IP multicast filtering is to optimize a switched network’s performance, so multicast
packets will only be forwarded to those ports containing multicast group hosts or multicast
routers/switches instead of flooding to all ports in the subnet (VLAN).
The switch with its IP multicast filtering capabili ty, not only passively monitors IGMP Query and
Report messages; it can also actively send IGMP Query messages to learn locations of multicast
routers/switches and member hosts in multicast groups within each VLAN.
However, note that IGMP neither alters nor routes any IP multicast packets. Since IGMP is not
concerned with the delivery of IP multicast packets across subnetworks, an external IP multicast
router is needed if IP multicast packets have to be routed across different subnetworks.