Moxa Technologies EDS-518A Series - Manual (Page 57)

EDS-518A Series User’s Manual Featured Functions
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Query Mode
Query mode allows the EDS-518A to work as the Querier if it has the lowest IP address on the
subnetwork to which it belongs. IGMP querying is enabled by default on the EDS-518A to help
prevent interoperability issues with some multicast routers that may not follow the lowest IP
address election method. Enable query mode to run multicast sessions on a network that does not
contain IGMP routers (or queriers).
NOTE EDS-518A is compatible with any device that conforms to the IGMP v2 and IGMP v3 device
protocol.
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)
EDS-518A 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. MOXA EDS-518A supports adding multicast groups manually to enable multicast
filtering.