Allied Telesis AT-WL2411 - Manual (Page 93)

AT-WL2411 Version 1.80 Installation and User’s Guide
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Internet Group
Management
Protocol (IGMP)
IGMP lets you originate multiple IP tunnels using a single IP multicast
address. Note that IGMP is independent of IP; it can be used to facilitate
multicast for IP or any other application.
IP routers only forward multicast packets to those subnets that have IP
hosts that participate in the respective IP multicast group. An IP host
uses IGMP to notify IP routers that it wants to participate in an IP
multicast group. Access points can act as IP hosts and participate in an IP
multicast group by enabling IGMP and by defining a Class D IP multicast
address. The Internet Assigned Numbers Authority has allocated
224.0.1.65 as an inter-access-point protocol (IAPP). You must enter this
address in the IP address list in the root access point. (Note that the
address list may contain other IP addresses.) and in the Multicast
Address field in the other access points.
If you enable IGMP on the root access point, the root access point uses a
Class D IP multicast address to send IP hello packets through IP routers
to access points on other subnets. If you enable IGMP on remote IP
subnets, intermediate IP routers will forward the IP hello packets to
those subnets. Enabling IGMP also has these advantages:
❑ Causes IP hello packets to be forwarded only to those subnets that
participate in the IP multicast group.
❑ Increases redundancy because multiple access points on a remote
subnet can receive IP hello packets.
IP multicast provides an ideal way to distribute IP hello messages.
Normally, you should enable IGMP and configure an IP multicast address
in at least one access point on each remote IP subnet. (Some routers can
provide proxy IGMP services for IP hosts.) IP multicast has the following
advantages:
❑ The user does not have to know unicast or directed broadcast IP
addresses in advance.
❑ IP multicast provides better built-in redundancy than IP unicast,
because any access point can establish an IP tunnel.
IP hello messages are only forwarded to those IP subnets and IP hosts
(such as access points) that participate in the multicast group. Directed
broadcast packets are forwarded to all IP hosts on the target subnet.