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PT-7728 User’s Manual Featured Functions
3-47
IGMP Multicast Filtering
IGMP is used by IP-supporting network devices to register hosts with multicast groups. It can be
used on all LANs and VLANs that contain a multicast capable IP router, and on other network
devices that support multicast filtering. IGMP works as follows:
1. The IP router (or querier) periodically sends query packets to all end-stations on the LANs or
VLANs that are connected to it. For networks with more than one IP router, the router with
the lowest IP address is the querier. A switch with IP address lower than the IP address of any
other IGMP queriers connected to the LAN or VLAN can become the IGMP querier.
2. When an IP host receives a query packet, it sends a report packet back that identifies the
multicast group that the end-station would like to join.
3. When the report packet arrives at a port on a switch with IGMP Snooping enabled, the switch
knows that the port should forward traffic for the multicast group, and then proceeds to
forward the packet to the router.
4. When the router receives the report packet, it registers that the LAN or VLAN requires traffic
for the multicast groups.
5. When the router forwards traffic for the multicast group to the LAN or VLAN, the switches
only forward the traffic to ports that received a report packet.
GMRP (GARP Multicast Registration Protocol)
The PT-7728 supports IEEE 802.1D-1998 GMRP (GARP Multicast Registration Protocol), which
differs from IGMP (Internet Group Management Protocol). GMRP is a MAC-based multicast
management protocol, whereas IGMP is IP-based. GMRP provides a mechanism that allows
bridges and end stations to register or de-register Group membership information dynamically.
GMRP functions similarly to GVRP, except that GMRP registers multicast addresses on ports.
When a port receives a GMRP-join message, it will register the multicast address to its database if
the multicast address is not registered, and all the multicast packets with that multicast address are
able to be forwarded from this port. When a port receives a GMRP-leave message, it will
de-register the multicast address from its database, and all the multicast packets with this multicast
address are not able to be forwarded from this port.
Static Multicast MAC
Some devices may only support multicast packets, but not support either IGMP Snooping or
GMRP. The PT-7728 supports adding multicast groups manually to enable multicast filtering.
Enabling Multicast Filtering
Use the serial console or Web interface to enable or disable IGMP Snooping and IGMP querying.
If IGMP Snooping is not enabled, then IP multicast traffic is always forwarded, flooding the
network.
Configuring IGMP Snooping
IGMP Snooping provides the ability to prune multicast traffic so that it travels only to those end
destinations that require that traffic, thereby reducing the amount of traffic on the Ethernet LAN.