Moxa Technologies PT-7728 Configuring IGMP Snooping

PT-7728 User’s Manual Featured Functions

3-47

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.

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.

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.

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.