6381-A4 Router Users Guide
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IGMP Proxy
Multicasting is a form of limited broadcast. UDP is used to send datagrams to all hosts that
belong to what is called a Host Group. A host group is a set of one or more hosts identified by a
single IP destination address.
Internet Group Management Protocol (IGMP) Proxy allows for forwarding of multicast traffic
between networks. Unlike broadcast which sends traffic to all possible addresses (and because it
requires duplication and transmission broadcasts may require a great deal of computation time
from the sending device), multicast provides a mechanism so data can be sent to a limited
number of devices. Unlike sending multiple normal unicast transmissions, which send
transmissions a single specific device (then many times over), multicast provides an option which
does not require many transmissions to be sent. Multicast has a group membership mechanism
where one data stream can be received by more than one device, so multicast does not require
the network bandwidth of multiple unicast transmissions.
Multicasting is useful when the same data needs to be sent to more than one device. For
instance, if one device is responsible for acquiring data that many other devices need, then
multicasting is a natural fit. Note that using multicasting as opposed to sending the same data to
individual devices uses less network bandwidth. The multicast feature also enables you to receive
multicast video streams from multicast servers.
With multicast, datagrams are sent to all hosts in a Host Group. A host group is a set of one or
more hosts identified by a single IP destination address. Host groups follow these standards:
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Anyone can join or leave a host group at will.
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There are no restrictions on a host’s location.
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There are no restrictions on the number of members that may belong to a host group.
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A host may belong to multiple host groups.
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Non-group members may send UDP datagrams to the host group.
Multicast provides a means for devices in host groups to get the datagrams from the host group
IP address. Multicast also enables you to receive multicast video streams from multicast servers.
IP hosts use IGMP to report their multicast group memberships to neighboring routers. Similarly,
multicast routers use IGMP to discover which of their hosts belong to multicast groups. Your 6381
supports IGMP proxy that handles IGMP messages. When enabled, your 6381 acts as a proxy
for a LAN host making requests to join and leave multicast groups, or a multicast router sending
multicast packets to multicast groups on the WAN side.
On a Join, the proxy sets up a multicast route for the interface and PC requesting the video
content. It then forwards the Join to the upstream multicast router. The Multicast IP traffic will then
be forwarded to the requesting device. Multicast traffic does not pass trough the Firewall or NAT.
On a leave, the Proxy removes the route and then forwards the leave to the upstream Multicast
router.
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