ADTRAN Stub Routing manual IP Multicast in Stub Topologies

IP Multicast in Stub Topologies IP Multicast Stub Routing in AOS

IP Multicast in Stub Topologies

A stub network is a portion of a network with a single connection to the remainder of the network. The spokes in a hub-and-spoke network are a common form of stub network, although there are other forms. In the stub portion of a network, a multicast routing protocol is not necessary for IP multicast operation. Since there is a single link, there is a single best path and no potential for a loop. All that is needed is a way to signal to the multicast network when there are active group members within the stub, allowing multicast streams to be sent to the stub network only when needed.

Referring back to Figure 2 on page 5, the satellite classrooms have a single connection to the multicast-enabled backbone, making them a stub network. When a student logs into PC1 and selects a class from the Media Server web interface, the following actions occur:

1.PC1 sends an IGMP message on its LAN indicating that it wants to join the multicast group for that class (let’s say address 224.1.1.1).

2.Satellite Router 1 (the stub router) registers the group membership on that interface.

3.Using its multicast routing protocol, Satellite Router 1 signals to the backbone router that it needs to receive packets to that group.

4.This signaling propagates to other routers (depending on the exact multicast routing protocol in use) and eventually to the Central Router.

5.IP packets from the Media Server to the group address 224.1.1.1 then flow through the backbone and ultimately to PC1.

While the multicast routing protocol does not consume significantly more bandwidth relative to other solutions, it does consume CPU and memory resources on the stub router even though it is performing only a portion of its total job, due to being a stub link.

An alternative would be to configure the satellite routers to proxy the IGMP messages from the classroom PCs to the multicast network. This alternative setup is described in the next section.

IGMP-Based Multicast Forwarding (IGMP Proxying)

Internet Draft draft-ietf-magma-igmp-proxy-06.txtspecifies a technique for performing just such a proxy function.

Terminology is tricky in this area. Other vendors support this technique, but some refer to it as Stub Multicast Routing where the stub router is configured as an IGMP Proxy Agent. Some vendors have another feature called IGMP Proxy which is an entirely different capability and not directly related to this application.

The following key concepts and terminology need to be understood regarding IGMP Forwarding.

•The stub router is typically the router at the stub location that connects to the remainder of the network. In Figure 2 on page 5, the satellite routers are the stub routers.

•When discussing multicast, it is common for the words upstream and downstream to be used relative to the distribution tree and the flow of a particular multicast stream. Regarding IGMP forwarding, it is important to note that these words are used relative to the stub router’s connection

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