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Multicast forwarding overview

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Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Multicast forwarding overview

With multicast forwarding, a router forwards multicast traffic to networks where other multicast devices are listening. Multicast forwarding prevents the forwarding of multicast traffic to networks where there are no nodes listening.

For multicast forwarding to work across an internetwork, nodes and routers must be multicast-capable.

Multicast-capable nodes

A multicast-capable node must be able to:

  • Send and receive multicast packets.

  • Register the multicast addresses being listened to by the node with local routers, so that multicast packets can be forwarded to the network of the node.

All computers running a member of the Microsoft® Windows Server 2003 family are IP multicast-capable and can both send and receive IP multicast traffic. IP multicasting applications that send multicast traffic must construct IP packets with the appropriate IP multicast address as the destination IP address. IP multicasting applications that receive multicast traffic must inform the TCP/IP protocol that they are listening for all traffic to a specified IP multicast address.

IP nodes use the Internet Group Management Protocol (IGMP) to register their interest in receiving IP multicast traffic from IP routers. IP nodes that use IGMP send out an IGMP Membership Report message to notify their local routers that they are listening on a specific IP multicast address.

Multicast-capable routers

A multicast-capable router must be able to:

  • Listen for all multicast traffic on all attached networks. Upon receiving multicast traffic, forward the multicast packet to attached networks where nodes are listening or where downstream routers have nodes that are listening.

    In the Windows Server 2003 family, the ability to listen for all multicast traffic and forward multicast packets is provided by the TCP/IP protocol. The TCP/IP protocol uses a multicast forwarding table to make decisions on where to forward incoming multicast traffic.

  • Listen for IGMP Membership Report messages and update the TCP/IP multicast forwarding table.

    In the Windows Server 2003 family, the ability to listen for IGMP Membership Report messages and update the TCP/IP multicast forwarding table is provided by the IGMP routing protocol on an interface operating in IGMP router mode.

  • Use a multicast routing protocol to propagate multicast group listening information to other multicast-capable routers.

    Routing and Remote Access does not provide any multicast routing protocols. However, the Routing and Remote Access service is an extensible platform and can support multicast routing protocols.

Note

  • The IGMP routing protocol component provided with Routing and Remote Access is not a multicast routing protocol. The maintenance of entries in the IP multicast forwarding table is a function of IGMP.

The IGMP routing protocol component

Maintaining entries in the TCP/IP multicast forwarding table is implemented through the IGMP routing protocol component, a component that is added as an IP routing protocol by using Routing and Remote Access. Once the IGMP routing protocol component is added, router interfaces are added to IGMP. You can configure each interface added to the IGMP routing protocol component in one of two operating modes:

  1. IGMP router mode

  2. IGMP proxy mode

IGMP router mode

In the Windows Server 2003 family, the ability to listen for IGMP Membership Report packets and track group membership is provided by an interface running in IGMP router mode. You must enable IGMP router mode on the interfaces where listening multicast hosts are located.

For more information about the IGMP router mode, see Multicast forwarding.

IGMP proxy mode

An interface running in IGMP proxy mode acts as a proxy multicast host that sends IGMP Membership Report messages on one interface for IGMP Membership Report messages received on all other interfaces running in IGMP router mode. The upstream router attached to the network of the IGMP proxy mode interface receives the IGMP proxy mode Membership Report packets and adds them to its own multicast tables. In this way, the upstream router knows to forward multicast packets to the network segment of the IGMP proxy mode interface for multicast groups that were registered by hosts attached to network segments of the IGMP proxy mode router.

When the upstream router forwards multicast traffic to the IGMP proxy mode interface network, it is forwarded to the appropriate hosts on the IGMP router mode interface networks by the TCP/IP protocol.

All nonlocal multicast traffic received on all interfaces running IGMP router mode is forwarded using the IGMP proxy mode interface. The upstream router that receives the forwarded multicast traffic can elect to forward the multicast traffic or discard it. Using IGMP proxy mode, multicast sources on networks attached to the server running Routing and Remote Access can send multicast traffic to multicast hosts attached to upstream multicast routers.

IGMP proxy mode is designed to pass IGMP Membership Report messages from a single router intranet to a multicast-capable portion of the Internet. The multicast-capable portion of the Internet is known as the Internet multicast backbone, or MBone. With IGMP proxy mode enabled on the Internet interface, hosts on the single router intranet can receive multicast traffic from multicast sources on the Internet and send multicast traffic to hosts on the Internet.

For more information about using IGMP proxy mode interfaces, see Multicast routing.