Remote Access Server and IP Multicast Support

The Windows 2000 remote access server also supports the forwarding of IP multicast traffic between remote access clients and the networks to which the remote access server is attached.

IP multicast support for remote access clients requires the following three elements, as illustrated in Figure 7.17.

1. Internet Group Management Protocol (IGMP) router mode is enabled on the interface connected to all of the remote access clients. In the Routing and Remote Access snap-in, this is the Internal interface.

2. IGMP proxy mode is enabled on a single interface.

3. The network corresponding to the interface on which IGMP proxy mode is enabled is part of an IP multicast–enabled network. An IP multicast–enabled network uses multicast routing protocols to propagate IP multicast traffic from multicast sources located on any network, to hosts located on any network. For example, the IP multicast-enabled portion of the Internet is called the Multicast Backbone or MBone .

Figure 7.17 Multicast Support for Remote Access

For more information about IP multicasting and its support in Windows 2000 Server, see "IP Multicast Support" in this book.

Note

Depending on your choices when running the Routing and Remote Access Server Setup Wizard, IGMP router mode and IGMP proxy mode may already be enabled on the appropriate interfaces.

Multicast Traffic to Remote Access Clients

To facilitate the forwarding of IP multicast traffic from multicast sources on the IP multicast–enabled network to remote access clients:

1. The multicast groups being listened to by remote access clients must be registered with the IP multicast routers of the IP multicast-enabled network.

2. IP multicast traffic from multicast sources must be forwarded to the remote access clients.

Remote Access Client Group Registration

Remote access clients register the IP multicast addresses from which they want to receive traffic by sending IGMP Membership Report messages across the remote access connection. The remote access server records the multicast groups registered by each remote access client and then forwards the IGMP Membership Report message using the interface on which IGMP proxy mode is enabled.

The forwarded IGMP Membership Report message is received by the IP multicast routers attached to the remote access server's network segment. The IP multicast routers of the IP multicast–enabled network use multicast routing protocols to create entries in their multicast forwarding tables, so that multicast traffic sent to the multicast groups — registered by the remote access clients — are forwarded to the network segment of the remote access server.

Forwarding Multicast Traffic

When the multicast source sends multicast traffic to the multicast group registered by the remote access clients, IP multicast routers forward the multicast traffic to the network segment of the remote access server for the interface on which IGMP proxy mode is enabled.

When the remote access server receives multicast traffic on the interface on which IGMP proxy mode is enabled, the multicast traffic is checked to determine whether any connected remote access client has registered an IGMP Membership Report for that multicast group. If the multicast traffic corresponds to a multicast group registered by a remote access client, the multicast traffic is forwarded to the remote access client.

Multicast Traffic from Remote Access Clients

To facilitate the forwarding of IP multicast traffic from remote access clients that are multicast sources:

1. The multicast groups being listened to by hosts must be registered with the IP multicast routers of the IP multicast–enabled network.

2. IP multicast traffic from the remote access clients must be forwarded to the group members.

Host Group Registration

Hosts on the IP multicast–enabled network register the IP multicast addresses for which they want to receive traffic from by sending IGMP Membership Report messages on their local network segments. The IP multicast routers of the IP multicast–enabled network use multicast routing protocols to create entries in their multicast forwarding tables. Therefore, multicast traffic sent to the multicast groups registered by the hosts are forwarded to the host's network segment.

Forwarding Multicast Traffic

When the remote access client sends multicast traffic across the remote access connection, the multicast traffic is forwarded to the network segment of the interface on the remote access server enabled for IGMP proxy mode. IP multicast routers on that network segment receive the forwarded multicast traffic and forward it to the network segments of the group members.

Additionally, the remote access server forwards the IP multicast traffic to other remote access clients that are listening for the IP multicast traffic of the remote access client that is the multicast source.

Internet-Based IP Multicast Traffic

If the remote access server is being used to provide Internet access to dial-up clients, then the following configuration allows IP multicast traffic to and from connected remote access clients:

1. The remote access server has a direct connection to the Internet's MBone or an indirect connection to the MBone through a logical tunnel.

2. The interface corresponding to the direct or indirect connection to the MBone is added to the IGMP routing protocol and enabled for IGMP proxy mode.

3. The Internal interface is added to the IGMP routing protocol and enabled for IGMP router mode.

Organization-Based IP Multicast Traffic

If the remote access server is being used to provide a connection to an organization's intranet to dial-up or VPN clients, then the following configuration allows IP multicast traffic to and from connected remote access clients:

1. The remote access server has a LAN interface on the organization's intranet, which is a network segment on the organization's IP multicast-enabled network.

2. The LAN interface connection to the organization intranet is added to the IGMP routing protocol and enabled for IGMP proxy mode.

3. The Internal interface is added to the IGMP routing protocol and enabled for IGMP router mode.